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- Committer: Bazaar Package Importer
- Author(s): Jelmer Vernooij
- Date: 2011-07-21 17:40:58 UTC
- mfrom: (1.1.12 upstream) (2.4.10 experimental)
- Revision ID: james.westby@ubuntu.com-20110721174058-byiuowgocek307cs
Tags: 1.5~pre2+git20110720-2
Fix dependency on pthreads when building on Linux 3.0.
- files added:
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- .pc/032_heimbase_pthread/cf
- doc/standardisation
- files removed:
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- files modified:
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doc/standardisation/draft-ietf-cat-kerberos-revisions-05.txt
1
INTERNET-DRAFT Clifford Neuman
2
John Kohl
3
Theodore Ts'o
4
March 10, 2000
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Expires September 10, 2000
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7
The Kerberos Network Authentication Service (V5)
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draft-ietf-cat-kerberos-revisions-05.txt
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STATUS OF THIS MEMO
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This document is an Internet-Draft and is in full conformance with all
13
provisions of Section 10 of RFC 2026. Internet-Drafts are working documents
14
of the Internet Engineering Task Force (IETF), its areas, and its working
15
groups. Note that other groups may also distribute working documents as
16
Internet-Drafts.
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Internet-Drafts are draft documents valid for a maximum of six months and
19
may be updated, replaced, or obsoleted by other documents at any time. It is
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inappropriate to use Internet-Drafts as reference material or to cite them
21
other than as "work in progress."
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The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/ietf/1id-abstracts.txt
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26
The list of Internet-Draft Shadow Directories can be accessed at
27
http://www.ietf.org/shadow.html.
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To learn the current status of any Internet-Draft, please check the
30
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
32
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
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The distribution of this memo is unlimited. It is filed as
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draft-ietf-cat-kerberos-revisions-05.txt, and expires September 10, 2000.
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Please send comments to: krb-protocol@MIT.EDU
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ABSTRACT
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This document provides an overview and specification of Version 5 of the
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Kerberos protocol, and updates RFC1510 to clarify aspects of the protocol
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and its intended use that require more detailed or clearer explanation than
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was provided in RFC1510. This document is intended to provide a detailed
44
description of the protocol, suitable for implementation, together with
45
descriptions of the appropriate use of protocol messages and fields within
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those messages.
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This document is not intended to describe Kerberos to the end user, system
49
administrator, or application developer. Higher level papers describing
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Version 5 of the Kerberos system [NT94] and documenting version 4 [SNS88],
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are available elsewhere.
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OVERVIEW
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This INTERNET-DRAFT describes the concepts and model upon which the Kerberos
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network authentication system is based. It also specifies Version 5 of the
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Kerberos protocol.
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The motivations, goals, assumptions, and rationale behind most design
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decisions are treated cursorily; they are more fully described in a paper
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available in IEEE communications [NT94] and earlier in the Kerberos portion
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of the Athena Technical Plan [MNSS87]. The protocols have been a proposed
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standard and are being considered for advancement for draft standard through
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the IETF standard process. Comments are encouraged on the presentation, but
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only minor refinements to the protocol as implemented or extensions that fit
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within current protocol framework will be considered at this time.
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Requests for addition to an electronic mailing list for discussion of
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Kerberos, kerberos@MIT.EDU, may be addressed to kerberos-request@MIT.EDU.
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This mailing list is gatewayed onto the Usenet as the group
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comp.protocols.kerberos. Requests for further information, including
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documents and code availability, may be sent to info-kerberos@MIT.EDU.
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BACKGROUND
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The Kerberos model is based in part on Needham and Schroeder's trusted
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third-party authentication protocol [NS78] and on modifications suggested by
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Denning and Sacco [DS81]. The original design and implementation of Kerberos
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Versions 1 through 4 was the work of two former Project Athena staff
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members, Steve Miller of Digital Equipment Corporation and Clifford Neuman
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(now at the Information Sciences Institute of the University of Southern
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California), along with Jerome Saltzer, Technical Director of Project
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Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other members
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of Project Athena have also contributed to the work on Kerberos.
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Version 5 of the Kerberos protocol (described in this document) has evolved
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from Version 4 based on new requirements and desires for features not
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available in Version 4. The design of Version 5 of the Kerberos protocol was
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led by Clifford Neuman and John Kohl with much input from the community. The
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development of the MIT reference implementation was led at MIT by John Kohl
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and Theodore T'so, with help and contributed code from many others. Since
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RFC1510 was issued, extensions and revisions to the protocol have been
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proposed by many individuals. Some of these proposals are reflected in this
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document. Where such changes involved significant effort, the document cites
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the contribution of the proposer.
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Reference implementations of both version 4 and version 5 of Kerberos are
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publicly available and commercial implementations have been developed and
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are widely used. Details on the differences between Kerberos Versions 4 and
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5 can be found in [KNT92].
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1. Introduction
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Kerberos provides a means of verifying the identities of principals, (e.g. a
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workstation user or a network server) on an open (unprotected) network. This
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is accomplished without relying on assertions by the host operating system,
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without basing trust on host addresses, without requiring physical security
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of all the hosts on the network, and under the assumption that packets
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traveling along the network can be read, modified, and inserted at will[1].
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Kerberos performs authentication under these conditions as a trusted
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third-party authentication service by using conventional (shared secret key
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[2] cryptography. Kerberos extensions have been proposed and implemented
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that provide for the use of public key cryptography during certain phases of
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the authentication protocol. These extensions provide for authentication of
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users registered with public key certification authorities, and allow the
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system to provide certain benefits of public key cryptography in situations
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where they are needed.
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The basic Kerberos authentication process proceeds as follows: A client
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sends a request to the authentication server (AS) requesting 'credentials'
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for a given server. The AS responds with these credentials, encrypted in the
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client's key. The credentials consist of 1) a 'ticket' for the server and 2)
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a temporary encryption key (often called a "session key"). The client
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transmits the ticket (which contains the client's identity and a copy of the
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session key, all encrypted in the server's key) to the server. The session
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key (now shared by the client and server) is used to authenticate the
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client, and may optionally be used to authenticate the server. It may also
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be used to encrypt further communication between the two parties or to
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exchange a separate sub-session key to be used to encrypt further
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communication.
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Implementation of the basic protocol consists of one or more authentication
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servers running on physically secure hosts. The authentication servers
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maintain a database of principals (i.e., users and servers) and their secret
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keys. Code libraries provide encryption and implement the Kerberos protocol.
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In order to add authentication to its transactions, a typical network
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application adds one or two calls to the Kerberos library directly or
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through the Generic Security Services Application Programming Interface,
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GSSAPI, described in separate document. These calls result in the
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transmission of the necessary messages to achieve authentication.
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The Kerberos protocol consists of several sub-protocols (or exchanges).
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There are two basic methods by which a client can ask a Kerberos server for
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credentials. In the first approach, the client sends a cleartext request for
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a ticket for the desired server to the AS. The reply is sent encrypted in
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the client's secret key. Usually this request is for a ticket-granting
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ticket (TGT) which can later be used with the ticket-granting server (TGS).
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In the second method, the client sends a request to the TGS. The client uses
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the TGT to authenticate itself to the TGS in the same manner as if it were
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contacting any other application server that requires Kerberos
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authentication. The reply is encrypted in the session key from the TGT.
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Though the protocol specification describes the AS and the TGS as separate
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servers, they are implemented in practice as different protocol entry points
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within a single Kerberos server.
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Once obtained, credentials may be used to verify the identity of the
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principals in a transaction, to ensure the integrity of messages exchanged
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between them, or to preserve privacy of the messages. The application is
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free to choose whatever protection may be necessary.
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To verify the identities of the principals in a transaction, the client
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transmits the ticket to the application server. Since the ticket is sent "in
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the clear" (parts of it are encrypted, but this encryption doesn't thwart
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replay) and might be intercepted and reused by an attacker, additional
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information is sent to prove that the message originated with the principal
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to whom the ticket was issued. This information (called the authenticator)
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is encrypted in the session key, and includes a timestamp. The timestamp
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proves that the message was recently generated and is not a replay.
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Encrypting the authenticator in the session key proves that it was generated
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by a party possessing the session key. Since no one except the requesting
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principal and the server know the session key (it is never sent over the
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network in the clear) this guarantees the identity of the client.
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The integrity of the messages exchanged between principals can also be
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guaranteed using the session key (passed in the ticket and contained in the
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credentials). This approach provides detection of both replay attacks and
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message stream modification attacks. It is accomplished by generating and
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transmitting a collision-proof checksum (elsewhere called a hash or digest
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function) of the client's message, keyed with the session key. Privacy and
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integrity of the messages exchanged between principals can be secured by
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encrypting the data to be passed using the session key contained in the
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ticket or the subsession key found in the authenticator.
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The authentication exchanges mentioned above require read-only access to the
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Kerberos database. Sometimes, however, the entries in the database must be
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modified, such as when adding new principals or changing a principal's key.
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This is done using a protocol between a client and a third Kerberos server,
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the Kerberos Administration Server (KADM). There is also a protocol for
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maintaining multiple copies of the Kerberos database. Neither of these
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protocols are described in this document.
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1.1. Cross-Realm Operation
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The Kerberos protocol is designed to operate across organizational
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boundaries. A client in one organization can be authenticated to a server in
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another. Each organization wishing to run a Kerberos server establishes its
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own 'realm'. The name of the realm in which a client is registered is part
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of the client's name, and can be used by the end-service to decide whether
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to honor a request.
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By establishing 'inter-realm' keys, the administrators of two realms can
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allow a client authenticated in the local realm to prove its identity to
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servers in other realms[3]. The exchange of inter-realm keys (a separate key
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may be used for each direction) registers the ticket-granting service of
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each realm as a principal in the other realm. A client is then able to
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obtain a ticket-granting ticket for the remote realm's ticket-granting
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service from its local realm. When that ticket-granting ticket is used, the
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remote ticket-granting service uses the inter-realm key (which usually
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differs from its own normal TGS key) to decrypt the ticket-granting ticket,
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and is thus certain that it was issued by the client's own TGS. Tickets
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issued by the remote ticket-granting service will indicate to the
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end-service that the client was authenticated from another realm.
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A realm is said to communicate with another realm if the two realms share an
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inter-realm key, or if the local realm shares an inter-realm key with an
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intermediate realm that communicates with the remote realm. An
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authentication path is the sequence of intermediate realms that are
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transited in communicating from one realm to another.
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Realms are typically organized hierarchically. Each realm shares a key with
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its parent and a different key with each child. If an inter-realm key is not
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directly shared by two realms, the hierarchical organization allows an
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authentication path to be easily constructed. If a hierarchical organization
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is not used, it may be necessary to consult a database in order to construct
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an authentication path between realms.
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Although realms are typically hierarchical, intermediate realms may be
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bypassed to achieve cross-realm authentication through alternate
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authentication paths (these might be established to make communication
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between two realms more efficient). It is important for the end-service to
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know which realms were transited when deciding how much faith to place in
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the authentication process. To facilitate this decision, a field in each
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ticket contains the names of the realms that were involved in authenticating
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the client.
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The application server is ultimately responsible for accepting or rejecting
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authentication and should check the transited field. The application server
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may choose to rely on the KDC for the application server's realm to check
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the transited field. The application server's KDC will set the
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TRANSITED-POLICY-CHECKED flag in this case. The KDC's for intermediate
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realms may also check the transited field as they issue
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ticket-granting-tickets for other realms, but they are encouraged not to do
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so. A client may request that the KDC's not check the transited field by
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setting the DISABLE-TRANSITED-CHECK flag. KDC's are encouraged but not
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required to honor this flag.
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1.2. Authorization
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As an authentication service, Kerberos provides a means of verifying the
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identity of principals on a network. Authentication is usually useful
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primarily as a first step in the process of authorization, determining
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whether a client may use a service, which objects the client is allowed to
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access, and the type of access allowed for each. Kerberos does not, by
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itself, provide authorization. Possession of a client ticket for a service
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provides only for authentication of the client to that service, and in the
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absence of a separate authorization procedure, it should not be considered
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by an application as authorizing the use of that service.
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Such separate authorization methods may be implemented as application
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specific access control functions and may be based on files such as the
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application server, or on separately issued authorization credentials such
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as those based on proxies [Neu93], or on other authorization services.
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Separately authenticated authorization credentials may be embedded in a
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tickets authorization data when encapsulated by the kdc-issued authorization
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data element.
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Applications should not be modified to accept the mere issuance of a service
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ticket by the Kerberos server (even by a modified Kerberos server) as
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granting authority to use the service, since such applications may become
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vulnerable to the bypass of this authorization check in an environment if
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they interoperate with other KDCs or where other options for application
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authentication (e.g. the PKTAPP proposal) are provided.
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1.3. Environmental assumptions
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Kerberos imposes a few assumptions on the environment in which it can
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properly function:
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* 'Denial of service' attacks are not solved with Kerberos. There are
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places in these protocols where an intruder can prevent an application
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from participating in the proper authentication steps. Detection and
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solution of such attacks (some of which can appear to be nnot-uncommon
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'normal' failure modes for the system) is usually best left to the
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human administrators and users.
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* Principals must keep their secret keys secret. If an intruder somehow
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steals a principal's key, it will be able to masquerade as that
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principal or impersonate any server to the legitimate principal.
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* 'Password guessing' attacks are not solved by Kerberos. If a user
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chooses a poor password, it is possible for an attacker to successfully
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mount an offline dictionary attack by repeatedly attempting to decrypt,
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with successive entries from a dictionary, messages obtained which are
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encrypted under a key derived from the user's password.
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* Each host on the network must have a clock which is 'loosely
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synchronized' to the time of the other hosts; this synchronization is
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used to reduce the bookkeeping needs of application servers when they
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do replay detection. The degree of "looseness" can be configured on a
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per-server basis, but is typically on the order of 5 minutes. If the
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clocks are synchronized over the network, the clock synchronization
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protocol must itself be secured from network attackers.
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* Principal identifiers are not recycled on a short-term basis. A typical
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mode of access control will use access control lists (ACLs) to grant
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permissions to particular principals. If a stale ACL entry remains for
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a deleted principal and the principal identifier is reused, the new
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principal will inherit rights specified in the stale ACL entry. By not
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re-using principal identifiers, the danger of inadvertent access is
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removed.
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1.4. Glossary of terms
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Below is a list of terms used throughout this document.
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Authentication
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Verifying the claimed identity of a principal.
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Authentication header
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A record containing a Ticket and an Authenticator to be presented to a
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server as part of the authentication process.
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Authentication path
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A sequence of intermediate realms transited in the authentication
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process when communicating from one realm to another.
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Authenticator
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A record containing information that can be shown to have been recently
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generated using the session key known only by the client and server.
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Authorization
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The process of determining whether a client may use a service, which
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objects the client is allowed to access, and the type of access allowed
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for each.
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Capability
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A token that grants the bearer permission to access an object or
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service. In Kerberos, this might be a ticket whose use is restricted by
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the contents of the authorization data field, but which lists no
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network addresses, together with the session key necessary to use the
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ticket.
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Ciphertext
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The output of an encryption function. Encryption transforms plaintext
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into ciphertext.
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Client
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A process that makes use of a network service on behalf of a user. Note
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that in some cases a Server may itself be a client of some other server
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(e.g. a print server may be a client of a file server).
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Credentials
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A ticket plus the secret session key necessary to successfully use that
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ticket in an authentication exchange.
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KDC
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Key Distribution Center, a network service that supplies tickets and
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temporary session keys; or an instance of that service or the host on
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which it runs. The KDC services both initial ticket and ticket-granting
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ticket requests. The initial ticket portion is sometimes referred to as
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the Authentication Server (or service). The ticket-granting ticket
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portion is sometimes referred to as the ticket-granting server (or
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service).
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Kerberos
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Aside from the 3-headed dog guarding Hades, the name given to Project
393
Athena's authentication service, the protocol used by that service, or
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the code used to implement the authentication service.
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Plaintext
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The input to an encryption function or the output of a decryption
405
function. Decryption transforms ciphertext into plaintext.
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Principal
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A uniquely named client or server instance that participates in a
408
network communication.
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Principal identifier
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The name used to uniquely identify each different principal.
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Seal
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To encipher a record containing several fields in such a way that the
413
fields cannot be individually replaced without either knowledge of the
414
encryption key or leaving evidence of tampering.
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Secret key
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An encryption key shared by a principal and the KDC, distributed
417
outside the bounds of the system, with a long lifetime. In the case of
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a human user's principal, the secret key is derived from a password.
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Server
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A particular Principal which provides a resource to network clients.
421
The server is sometimes refered to as the Application Server.
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Service
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A resource provided to network clients; often provided by more than one
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server (for example, remote file service).
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Session key
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A temporary encryption key used between two principals, with a lifetime
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limited to the duration of a single login "session".
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Sub-session key
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A temporary encryption key used between two principals, selected and
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exchanged by the principals using the session key, and with a lifetime
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limited to the duration of a single association.
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Ticket
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A record that helps a client authenticate itself to a server; it
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contains the client's identity, a session key, a timestamp, and other
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information, all sealed using the server's secret key. It only serves
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to authenticate a client when presented along with a fresh
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Authenticator.
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2. Ticket flag uses and requests
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Each Kerberos ticket contains a set of flags which are used to indicate
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various attributes of that ticket. Most flags may be requested by a client
443
when the ticket is obtained; some are automatically turned on and off by a
444
Kerberos server as required. The following sections explain what the various
445
flags mean, and gives examples of reasons to use such a flag.
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2.1. Initial and pre-authenticated tickets
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The INITIAL flag indicates that a ticket was issued using the AS protocol
450
and not issued based on a ticket-granting ticket. Application servers that
451
want to require the demonstrated knowledge of a client's secret key (e.g. a
452
password-changing program) can insist that this flag be set in any tickets
453
they accept, and thus be assured that the client's key was recently
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presented to the application client.
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The PRE-AUTHENT and HW-AUTHENT flags provide addition information about the
457
initial authentication, regardless of whether the current ticket was issued
458
directly (in which case INITIAL will also be set) or issued on the basis of
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a ticket-granting ticket (in which case the INITIAL flag is clear, but the
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PRE-AUTHENT and HW-AUTHENT flags are carried forward from the
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ticket-granting ticket).
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2.2. Invalid tickets
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The INVALID flag indicates that a ticket is invalid. Application servers
474
must reject tickets which have this flag set. A postdated ticket will
475
usually be issued in this form. Invalid tickets must be validated by the KDC
476
before use, by presenting them to the KDC in a TGS request with the VALIDATE
477
option specified. The KDC will only validate tickets after their starttime
478
has passed. The validation is required so that postdated tickets which have
479
been stolen before their starttime can be rendered permanently invalid
480
(through a hot-list mechanism) (see section 3.3.3.1).
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2.3. Renewable tickets
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Applications may desire to hold tickets which can be valid for long periods
485
of time. However, this can expose their credentials to potential theft for
486
equally long periods, and those stolen credentials would be valid until the
487
expiration time of the ticket(s). Simply using short-lived tickets and
488
obtaining new ones periodically would require the client to have long-term
489
access to its secret key, an even greater risk. Renewable tickets can be
490
used to mitigate the consequences of theft. Renewable tickets have two
491
"expiration times": the first is when the current instance of the ticket
492
expires, and the second is the latest permissible value for an individual
493
expiration time. An application client must periodically (i.e. before it
494
expires) present a renewable ticket to the KDC, with the RENEW option set in
495
the KDC request. The KDC will issue a new ticket with a new session key and
496
a later expiration time. All other fields of the ticket are left unmodified
497
by the renewal process. When the latest permissible expiration time arrives,
498
the ticket expires permanently. At each renewal, the KDC may consult a
499
hot-list to determine if the ticket had been reported stolen since its last
500
renewal; it will refuse to renew such stolen tickets, and thus the usable
501
lifetime of stolen tickets is reduced.
502
503
The RENEWABLE flag in a ticket is normally only interpreted by the
504
ticket-granting service (discussed below in section 3.3). It can usually be
505
ignored by application servers. However, some particularly careful
506
application servers may wish to disallow renewable tickets.
507
508
If a renewable ticket is not renewed by its expiration time, the KDC will
509
not renew the ticket. The RENEWABLE flag is reset by default, but a client
510
may request it be set by setting the RENEWABLE option in the KRB_AS_REQ
511
message. If it is set, then the renew-till field in the ticket contains the
512
time after which the ticket may not be renewed.
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2.4. Postdated tickets
515
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Applications may occasionally need to obtain tickets for use much later,
517
e.g. a batch submission system would need tickets to be valid at the time
518
the batch job is serviced. However, it is dangerous to hold valid tickets in
519
a batch queue, since they will be on-line longer and more prone to theft.
520
Postdated tickets provide a way to obtain these tickets from the KDC at job
521
submission time, but to leave them "dormant" until they are activated and
522
validated by a further request of the KDC. If a ticket theft were reported
523
in the interim, the KDC would refuse to validate the ticket, and the thief
524
would be foiled.
525
526
The MAY-POSTDATE flag in a ticket is normally only interpreted by the
527
ticket-granting service. It can be ignored by application servers. This flag
528
must be set in a ticket-granting ticket in order to issue a postdated ticket
529
based on the presented ticket. It is reset by default; it may be requested
530
531
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532
533
534
535
536
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
537
538
by a client by setting the ALLOW-POSTDATE option in the KRB_AS_REQ message.
539
This flag does not allow a client to obtain a postdated ticket-granting
540
ticket; postdated ticket-granting tickets can only by obtained by requesting
541
the postdating in the KRB_AS_REQ message. The life (endtime-starttime) of a
542
postdated ticket will be the remaining life of the ticket-granting ticket at
543
the time of the request, unless the RENEWABLE option is also set, in which
544
case it can be the full life (endtime-starttime) of the ticket-granting
545
ticket. The KDC may limit how far in the future a ticket may be postdated.
546
547
The POSTDATED flag indicates that a ticket has been postdated. The
548
application server can check the authtime field in the ticket to see when
549
the original authentication occurred. Some services may choose to reject
550
postdated tickets, or they may only accept them within a certain period
551
after the original authentication. When the KDC issues a POSTDATED ticket,
552
it will also be marked as INVALID, so that the application client must
553
present the ticket to the KDC to be validated before use.
554
555
2.5. Proxiable and proxy tickets
556
557
At times it may be necessary for a principal to allow a service to perform
558
an operation on its behalf. The service must be able to take on the identity
559
of the client, but only for a particular purpose. A principal can allow a
560
service to take on the principal's identity for a particular purpose by
561
granting it a proxy.
562
563
The process of granting a proxy using the proxy and proxiable flags is used
564
to provide credentials for use with specific services. Though conceptually
565
also a proxy, user's wishing to delegate their identity for ANY purpose must
566
use the ticket forwarding mechanism described in the next section to forward
567
a ticket granting ticket.
568
569
The PROXIABLE flag in a ticket is normally only interpreted by the
570
ticket-granting service. It can be ignored by application servers. When set,
571
this flag tells the ticket-granting server that it is OK to issue a new
572
ticket (but not a ticket-granting ticket) with a different network address
573
based on this ticket. This flag is set if requested by the client on initial
574
authentication. By default, the client will request that it be set when
575
requesting a ticket granting ticket, and reset when requesting any other
576
ticket.
577
578
This flag allows a client to pass a proxy to a server to perform a remote
579
request on its behalf, e.g. a print service client can give the print server
580
a proxy to access the client's files on a particular file server in order to
581
satisfy a print request.
582
583
In order to complicate the use of stolen credentials, Kerberos tickets are
584
usually valid from only those network addresses specifically included in the
585
ticket[4]. When granting a proxy, the client must specify the new network
586
address from which the proxy is to be used, or indicate that the proxy is to
587
be issued for use from any address.
588
589
The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket.
590
Application servers may check this flag and at their option they may require
591
additional authentication from the agent presenting the proxy in order to
592
provide an audit trail.
593
594
2.6. Forwardable tickets
595
596
Authentication forwarding is an instance of a proxy where the service is
597
598
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599
600
601
602
603
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
604
605
granted complete use of the client's identity. An example where it might be
606
used is when a user logs in to a remote system and wants authentication to
607
work from that system as if the login were local.
608
609
The FORWARDABLE flag in a ticket is normally only interpreted by the
610
ticket-granting service. It can be ignored by application servers. The
611
FORWARDABLE flag has an interpretation similar to that of the PROXIABLE
612
flag, except ticket-granting tickets may also be issued with different
613
network addresses. This flag is reset by default, but users may request that
614
it be set by setting the FORWARDABLE option in the AS request when they
615
request their initial ticket- granting ticket.
616
617
This flag allows for authentication forwarding without requiring the user to
618
enter a password again. If the flag is not set, then authentication
619
forwarding is not permitted, but the same result can still be achieved if
620
the user engages in the AS exchange specifying the requested network
621
addresses and supplies a password.
622
623
The FORWARDED flag is set by the TGS when a client presents a ticket with
624
the FORWARDABLE flag set and requests a forwarded ticket by specifying the
625
FORWARDED KDC option and supplying a set of addresses for the new ticket. It
626
is also set in all tickets issued based on tickets with the FORWARDED flag
627
set. Application servers may choose to process FORWARDED tickets differently
628
than non-FORWARDED tickets.
629
630
2.7. Other KDC options
631
632
There are two additional options which may be set in a client's request of
633
the KDC. The RENEWABLE-OK option indicates that the client will accept a
634
renewable ticket if a ticket with the requested life cannot otherwise be
635
provided. If a ticket with the requested life cannot be provided, then the
636
KDC may issue a renewable ticket with a renew-till equal to the the
637
requested endtime. The value of the renew-till field may still be adjusted
638
by site-determined limits or limits imposed by the individual principal or
639
server.
640
641
The ENC-TKT-IN-SKEY option is honored only by the ticket-granting service.
642
It indicates that the ticket to be issued for the end server is to be
643
encrypted in the session key from the a additional second ticket-granting
644
ticket provided with the request. See section 3.3.3 for specific details.
645
646
3. Message Exchanges
647
648
The following sections describe the interactions between network clients and
649
servers and the messages involved in those exchanges.
650
651
3.1. The Authentication Service Exchange
652
653
Summary
654
Message direction Message type Section
655
1. Client to Kerberos KRB_AS_REQ 5.4.1
656
2. Kerberos to client KRB_AS_REP or 5.4.2
657
KRB_ERROR 5.9.1
658
659
The Authentication Service (AS) Exchange between the client and the Kerberos
660
Authentication Server is initiated by a client when it wishes to obtain
661
authentication credentials for a given server but currently holds no
662
credentials. In its basic form, the client's secret key is used for
663
encryption and decryption. This exchange is typically used at the initiation
664
665
Neuman, Ts'o, Kohl Expires: 10 September, 2000
666
667
668
669
670
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
671
672
of a login session to obtain credentials for a Ticket-Granting Server which
673
will subsequently be used to obtain credentials for other servers (see
674
section 3.3) without requiring further use of the client's secret key. This
675
exchange is also used to request credentials for services which must not be
676
mediated through the Ticket-Granting Service, but rather require a
677
principal's secret key, such as the password-changing service[5]. This
678
exchange does not by itself provide any assurance of the the identity of the
679
user[6].
680
681
The exchange consists of two messages: KRB_AS_REQ from the client to
682
Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these
683
messages are described in sections 5.4.1, 5.4.2, and 5.9.1.
684
685
In the request, the client sends (in cleartext) its own identity and the
686
identity of the server for which it is requesting credentials. The response,
687
KRB_AS_REP, contains a ticket for the client to present to the server, and a
688
session key that will be shared by the client and the server. The session
689
key and additional information are encrypted in the client's secret key. The
690
KRB_AS_REP message contains information which can be used to detect replays,
691
and to associate it with the message to which it replies. Various errors can
692
occur; these are indicated by an error response (KRB_ERROR) instead of the
693
KRB_AS_REP response. The error message is not encrypted. The KRB_ERROR
694
message contains information which can be used to associate it with the
695
message to which it replies. The lack of encryption in the KRB_ERROR message
696
precludes the ability to detect replays, fabrications, or modifications of
697
such messages.
698
699
Without preautentication, the authentication server does not know whether
700
the client is actually the principal named in the request. It simply sends a
701
reply without knowing or caring whether they are the same. This is
702
acceptable because nobody but the principal whose identity was given in the
703
request will be able to use the reply. Its critical information is encrypted
704
in that principal's key. The initial request supports an optional field that
705
can be used to pass additional information that might be needed for the
706
initial exchange. This field may be used for preauthentication as described
707
in section [hl<>].
708
709
3.1.1. Generation of KRB_AS_REQ message
710
711
The client may specify a number of options in the initial request. Among
712
these options are whether pre-authentication is to be performed; whether the
713
requested ticket is to be renewable, proxiable, or forwardable; whether it
714
should be postdated or allow postdating of derivative tickets; and whether a
715
renewable ticket will be accepted in lieu of a non-renewable ticket if the
716
requested ticket expiration date cannot be satisfied by a non-renewable
717
ticket (due to configuration constraints; see section 4). See section A.1
718
for pseudocode.
719
720
The client prepares the KRB_AS_REQ message and sends it to the KDC.
721
722
3.1.2. Receipt of KRB_AS_REQ message
723
724
If all goes well, processing the KRB_AS_REQ message will result in the
725
creation of a ticket for the client to present to the server. The format for
726
the ticket is described in section 5.3.1. The contents of the ticket are
727
determined as follows.
728
729
3.1.3. Generation of KRB_AS_REP message
730
731
732
Neuman, Ts'o, Kohl Expires: 10 September, 2000
733
734
735
736
737
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
738
739
The authentication server looks up the client and server principals named in
740
the KRB_AS_REQ in its database, extracting their respective keys. If
741
required, the server pre-authenticates the request, and if the
742
pre-authentication check fails, an error message with the code
743
KDC_ERR_PREAUTH_FAILED is returned. If the server cannot accommodate the
744
requested encryption type, an error message with code KDC_ERR_ETYPE_NOSUPP
745
is returned. Otherwise it generates a 'random' session key[7].
746
747
If there are multiple encryption keys registered for a client in the
748
Kerberos database (or if the key registered supports multiple encryption
749
types; e.g. DES-CBC-CRC and DES-CBC-MD5), then the etype field from the AS
750
request is used by the KDC to select the encryption method to be used for
751
encrypting the response to the client. If there is more than one supported,
752
strong encryption type in the etype list, the first valid etype for which an
753
encryption key is available is used. The encryption method used to respond
754
to a TGS request is taken from the keytype of the session key found in the
755
ticket granting ticket. [***I will change the example keytypes to be 3DES
756
based examples 7/14***]
757
758
When the etype field is present in a KDC request, whether an AS or TGS
759
request, the KDC will attempt to assign the type of the random session key
760
from the list of methods in the etype field. The KDC will select the
761
appropriate type using the list of methods provided together with
762
information from the Kerberos database indicating acceptable encryption
763
methods for the application server. The KDC will not issue tickets with a
764
weak session key encryption type.
765
766
If the requested start time is absent, indicates a time in the past, or is
767
within the window of acceptable clock skew for the KDC and the POSTDATE
768
option has not been specified, then the start time of the ticket is set to
769
the authentication server's current time. If it indicates a time in the
770
future beyond the acceptable clock skew, but the POSTDATED option has not
771
been specified then the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise
772
the requested start time is checked against the policy of the local realm
773
(the administrator might decide to prohibit certain types or ranges of
774
postdated tickets), and if acceptable, the ticket's start time is set as
775
requested and the INVALID flag is set in the new ticket. The postdated
776
ticket must be validated before use by presenting it to the KDC after the
777
start time has been reached.
778
779
The expiration time of the ticket will be set to the minimum of the
780
following:
781
782
* The expiration time (endtime) requested in the KRB_AS_REQ message.
783
* The ticket's start time plus the maximum allowable lifetime associated
784
with the client principal (the authentication server's database
785
includes a maximum ticket lifetime field in each principal's record;
786
see section 4).
787
* The ticket's start time plus the maximum allowable lifetime associated
788
with the server principal.
789
* The ticket's start time plus the maximum lifetime set by the policy of
790
the local realm.
791
792
If the requested expiration time minus the start time (as determined above)
793
is less than a site-determined minimum lifetime, an error message with code
794
KDC_ERR_NEVER_VALID is returned. If the requested expiration time for the
795
ticket exceeds what was determined as above, and if the 'RENEWABLE-OK'
796
option was requested, then the 'RENEWABLE' flag is set in the new ticket,
797
and the renew-till value is set as if the 'RENEWABLE' option were requested
798
799
Neuman, Ts'o, Kohl Expires: 10 September, 2000
800
801
802
803
804
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
805
806
(the field and option names are described fully in section 5.4.1).
807
808
If the RENEWABLE option has been requested or if the RENEWABLE-OK option has
809
been set and a renewable ticket is to be issued, then the renew-till field
810
is set to the minimum of:
811
812
* Its requested value.
813
* The start time of the ticket plus the minimum of the two maximum
814
renewable lifetimes associated with the principals' database entries.
815
* The start time of the ticket plus the maximum renewable lifetime set by
816
the policy of the local realm.
817
818
The flags field of the new ticket will have the following options set if
819
they have been requested and if the policy of the local realm allows:
820
FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE. If the new
821
ticket is post-dated (the start time is in the future), its INVALID flag
822
will also be set.
823
824
If all of the above succeed, the server formats a KRB_AS_REP message (see
825
section 5.4.2), copying the addresses in the request into the caddr of the
826
response, placing any required pre-authentication data into the padata of
827
the response, and encrypts the ciphertext part in the client's key using the
828
requested encryption method, and sends it to the client. See section A.2 for
829
pseudocode.
830
831
3.1.4. Generation of KRB_ERROR message
832
833
Several errors can occur, and the Authentication Server responds by
834
returning an error message, KRB_ERROR, to the client, with the error-code
835
and e-text fields set to appropriate values. The error message contents and
836
details are described in Section 5.9.1.
837
838
3.1.5. Receipt of KRB_AS_REP message
839
840
If the reply message type is KRB_AS_REP, then the client verifies that the
841
cname and crealm fields in the cleartext portion of the reply match what it
842
requested. If any padata fields are present, they may be used to derive the
843
proper secret key to decrypt the message. The client decrypts the encrypted
844
part of the response using its secret key, verifies that the nonce in the
845
encrypted part matches the nonce it supplied in its request (to detect
846
replays). It also verifies that the sname and srealm in the response match
847
those in the request (or are otherwise expected values), and that the host
848
address field is also correct. It then stores the ticket, session key, start
849
and expiration times, and other information for later use. The
850
key-expiration field from the encrypted part of the response may be checked
851
to notify the user of impending key expiration (the client program could
852
then suggest remedial action, such as a password change). See section A.3
853
for pseudocode.
854
855
Proper decryption of the KRB_AS_REP message is not sufficient to verify the
856
identity of the user; the user and an attacker could cooperate to generate a
857
KRB_AS_REP format message which decrypts properly but is not from the proper
858
KDC. If the host wishes to verify the identity of the user, it must require
859
the user to present application credentials which can be verified using a
860
securely-stored secret key for the host. If those credentials can be
861
verified, then the identity of the user can be assured.
862
863
3.1.6. Receipt of KRB_ERROR message
864
865
866
Neuman, Ts'o, Kohl Expires: 10 September, 2000
867
868
869
870
871
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
872
873
If the reply message type is KRB_ERROR, then the client interprets it as an
874
error and performs whatever application-specific tasks are necessary to
875
recover.
876
877
3.2. The Client/Server Authentication Exchange
878
879
Summary
880
Message direction Message type Section
881
Client to Application server KRB_AP_REQ 5.5.1
882
[optional] Application server to client KRB_AP_REP or 5.5.2
883
KRB_ERROR 5.9.1
884
885
The client/server authentication (CS) exchange is used by network
886
applications to authenticate the client to the server and vice versa. The
887
client must have already acquired credentials for the server using the AS or
888
TGS exchange.
889
890
3.2.1. The KRB_AP_REQ message
891
892
The KRB_AP_REQ contains authentication information which should be part of
893
the first message in an authenticated transaction. It contains a ticket, an
894
authenticator, and some additional bookkeeping information (see section
895
5.5.1 for the exact format). The ticket by itself is insufficient to
896
authenticate a client, since tickets are passed across the network in
897
cleartext[DS90], so the authenticator is used to prevent invalid replay of
898
tickets by proving to the server that the client knows the session key of
899
the ticket and thus is entitled to use the ticket. The KRB_AP_REQ message is
900
referred to elsewhere as the 'authentication header.'
901
902
3.2.2. Generation of a KRB_AP_REQ message
903
904
When a client wishes to initiate authentication to a server, it obtains
905
(either through a credentials cache, the AS exchange, or the TGS exchange) a
906
ticket and session key for the desired service. The client may re-use any
907
tickets it holds until they expire. To use a ticket the client constructs a
908
new Authenticator from the the system time, its name, and optionally an
909
application specific checksum, an initial sequence number to be used in
910
KRB_SAFE or KRB_PRIV messages, and/or a session subkey to be used in
911
negotiations for a session key unique to this particular session.
912
Authenticators may not be re-used and will be rejected if replayed to a
913
server[LGDSR87]. If a sequence number is to be included, it should be
914
randomly chosen so that even after many messages have been exchanged it is
915
not likely to collide with other sequence numbers in use.
916
917
The client may indicate a requirement of mutual authentication or the use of
918
a session-key based ticket by setting the appropriate flag(s) in the
919
ap-options field of the message.
920
921
The Authenticator is encrypted in the session key and combined with the
922
ticket to form the KRB_AP_REQ message which is then sent to the end server
923
along with any additional application-specific information. See section A.9
924
for pseudocode.
925
926
3.2.3. Receipt of KRB_AP_REQ message
927
928
Authentication is based on the server's current time of day (clocks must be
929
loosely synchronized), the authenticator, and the ticket. Several errors are
930
possible. If an error occurs, the server is expected to reply to the client
931
with a KRB_ERROR message. This message may be encapsulated in the
932
933
Neuman, Ts'o, Kohl Expires: 10 September, 2000
934
935
936
937
938
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
939
940
application protocol if its 'raw' form is not acceptable to the protocol.
941
The format of error messages is described in section 5.9.1.
942
943
The algorithm for verifying authentication information is as follows. If the
944
message type is not KRB_AP_REQ, the server returns the KRB_AP_ERR_MSG_TYPE
945
error. If the key version indicated by the Ticket in the KRB_AP_REQ is not
946
one the server can use (e.g., it indicates an old key, and the server no
947
longer possesses a copy of the old key), the KRB_AP_ERR_BADKEYVER error is
948
returned. If the USE-SESSION-KEY flag is set in the ap-options field, it
949
indicates to the server that the ticket is encrypted in the session key from
950
the server's ticket-granting ticket rather than its secret key[10]. Since it
951
is possible for the server to be registered in multiple realms, with
952
different keys in each, the srealm field in the unencrypted portion of the
953
ticket in the KRB_AP_REQ is used to specify which secret key the server
954
should use to decrypt that ticket. The KRB_AP_ERR_NOKEY error code is
955
returned if the server doesn't have the proper key to decipher the ticket.
956
957
The ticket is decrypted using the version of the server's key specified by
958
the ticket. If the decryption routines detect a modification of the ticket
959
(each encryption system must provide safeguards to detect modified
960
ciphertext; see section 6), the KRB_AP_ERR_BAD_INTEGRITY error is returned
961
(chances are good that different keys were used to encrypt and decrypt).
962
963
The authenticator is decrypted using the session key extracted from the
964
decrypted ticket. If decryption shows it to have been modified, the
965
KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm of the client
966
from the ticket are compared against the same fields in the authenticator.
967
If they don't match, the KRB_AP_ERR_BADMATCH error is returned (they might
968
not match, for example, if the wrong session key was used to encrypt the
969
authenticator). The addresses in the ticket (if any) are then searched for
970
an address matching the operating-system reported address of the client. If
971
no match is found or the server insists on ticket addresses but none are
972
present in the ticket, the KRB_AP_ERR_BADADDR error is returned.
973
974
If the local (server) time and the client time in the authenticator differ
975
by more than the allowable clock skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW
976
error is returned. If the server name, along with the client name, time and
977
microsecond fields from the Authenticator match any recently-seen such
978
tuples, the KRB_AP_ERR_REPEAT error is returned[11]. The server must
979
remember any authenticator presented within the allowable clock skew, so
980
that a replay attempt is guaranteed to fail. If a server loses track of any
981
authenticator presented within the allowable clock skew, it must reject all
982
requests until the clock skew interval has passed. This assures that any
983
lost or re-played authenticators will fall outside the allowable clock skew
984
and can no longer be successfully replayed (If this is not done, an attacker
985
could conceivably record the ticket and authenticator sent over the network
986
to a server, then disable the client's host, pose as the disabled host, and
987
replay the ticket and authenticator to subvert the authentication.). If a
988
sequence number is provided in the authenticator, the server saves it for
989
later use in processing KRB_SAFE and/or KRB_PRIV messages. If a subkey is
990
present, the server either saves it for later use or uses it to help
991
generate its own choice for a subkey to be returned in a KRB_AP_REP message.
992
993
The server computes the age of the ticket: local (server) time minus the
994
start time inside the Ticket. If the start time is later than the current
995
time by more than the allowable clock skew or if the INVALID flag is set in
996
the ticket, the KRB_AP_ERR_TKT_NYV error is returned. Otherwise, if the
997
current time is later than end time by more than the allowable clock skew,
998
the KRB_AP_ERR_TKT_EXPIRED error is returned.
999
1000
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1001
1002
1003
1004
1005
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1006
1007
1008
If all these checks succeed without an error, the server is assured that the
1009
client possesses the credentials of the principal named in the ticket and
1010
thus, the client has been authenticated to the server. See section A.10 for
1011
pseudocode.
1012
1013
Passing these checks provides only authentication of the named principal; it
1014
does not imply authorization to use the named service. Applications must
1015
make a separate authorization decisions based upon the authenticated name of
1016
the user, the requested operation, local acces control information such as
1017
that contained in a .k5login or .k5users file, and possibly a separate
1018
distributed authorization service.
1019
1020
3.2.4. Generation of a KRB_AP_REP message
1021
1022
Typically, a client's request will include both the authentication
1023
information and its initial request in the same message, and the server need
1024
not explicitly reply to the KRB_AP_REQ. However, if mutual authentication
1025
(not only authenticating the client to the server, but also the server to
1026
the client) is being performed, the KRB_AP_REQ message will have
1027
MUTUAL-REQUIRED set in its ap-options field, and a KRB_AP_REP message is
1028
required in response. As with the error message, this message may be
1029
encapsulated in the application protocol if its "raw" form is not acceptable
1030
to the application's protocol. The timestamp and microsecond field used in
1031
the reply must be the client's timestamp and microsecond field (as provided
1032
in the authenticator)[12]. If a sequence number is to be included, it should
1033
be randomly chosen as described above for the authenticator. A subkey may be
1034
included if the server desires to negotiate a different subkey. The
1035
KRB_AP_REP message is encrypted in the session key extracted from the
1036
ticket. See section A.11 for pseudocode.
1037
1038
3.2.5. Receipt of KRB_AP_REP message
1039
1040
If a KRB_AP_REP message is returned, the client uses the session key from
1041
the credentials obtained for the server[13] to decrypt the message, and
1042
verifies that the timestamp and microsecond fields match those in the
1043
Authenticator it sent to the server. If they match, then the client is
1044
assured that the server is genuine. The sequence number and subkey (if
1045
present) are retained for later use. See section A.12 for pseudocode.
1046
1047
3.2.6. Using the encryption key
1048
1049
After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and server
1050
share an encryption key which can be used by the application. The 'true
1051
session key' to be used for KRB_PRIV, KRB_SAFE, or other
1052
application-specific uses may be chosen by the application based on the
1053
subkeys in the KRB_AP_REP message and the authenticator[14]. In some cases,
1054
the use of this session key will be implicit in the protocol; in others the
1055
method of use must be chosen from several alternatives. We leave the
1056
protocol negotiations of how to use the key (e.g. selecting an encryption or
1057
checksum type) to the application programmer; the Kerberos protocol does not
1058
constrain the implementation options, but an example of how this might be
1059
done follows.
1060
1061
One way that an application may choose to negotiate a key to be used for
1062
subequent integrity and privacy protection is for the client to propose a
1063
key in the subkey field of the authenticator. The server can then choose a
1064
key using the proposed key from the client as input, returning the new
1065
subkey in the subkey field of the application reply. This key could then be
1066
1067
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1068
1069
1070
1071
1072
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1073
1074
used for subsequent communication. To make this example more concrete, if
1075
the encryption method in use required a 56 bit key, and for whatever reason,
1076
one of the parties was prevented from using a key with more than 40 unknown
1077
bits, this method would allow the the party which is prevented from using
1078
more than 40 bits to either propose (if the client) an initial key with a
1079
known quantity for 16 of those bits, or to mask 16 of the bits (if the
1080
server) with the known quantity. The application implementor is warned,
1081
however, that this is only an example, and that an analysis of the
1082
particular crytosystem to be used, and the reasons for limiting the key
1083
length, must be made before deciding whether it is acceptable to mask bits
1084
of the key.
1085
1086
With both the one-way and mutual authentication exchanges, the peers should
1087
take care not to send sensitive information to each other without proper
1088
assurances. In particular, applications that require privacy or integrity
1089
should use the KRB_AP_REP response from the server to client to assure both
1090
client and server of their peer's identity. If an application protocol
1091
requires privacy of its messages, it can use the KRB_PRIV message (section
1092
3.5). The KRB_SAFE message (section 3.4) can be used to assure integrity.
1093
1094
3.3. The Ticket-Granting Service (TGS) Exchange
1095
1096
Summary
1097
Message direction Message type Section
1098
1. Client to Kerberos KRB_TGS_REQ 5.4.1
1099
2. Kerberos to client KRB_TGS_REP or 5.4.2
1100
KRB_ERROR 5.9.1
1101
1102
The TGS exchange between a client and the Kerberos Ticket-Granting Server is
1103
initiated by a client when it wishes to obtain authentication credentials
1104
for a given server (which might be registered in a remote realm), when it
1105
wishes to renew or validate an existing ticket, or when it wishes to obtain
1106
a proxy ticket. In the first case, the client must already have acquired a
1107
ticket for the Ticket-Granting Service using the AS exchange (the
1108
ticket-granting ticket is usually obtained when a client initially
1109
authenticates to the system, such as when a user logs in). The message
1110
format for the TGS exchange is almost identical to that for the AS exchange.
1111
The primary difference is that encryption and decryption in the TGS exchange
1112
does not take place under the client's key. Instead, the session key from
1113
the ticket-granting ticket or renewable ticket, or sub-session key from an
1114
Authenticator is used. As is the case for all application servers, expired
1115
tickets are not accepted by the TGS, so once a renewable or ticket-granting
1116
ticket expires, the client must use a separate exchange to obtain valid
1117
tickets.
1118
1119
The TGS exchange consists of two messages: A request (KRB_TGS_REQ) from the
1120
client to the Kerberos Ticket-Granting Server, and a reply (KRB_TGS_REP or
1121
KRB_ERROR). The KRB_TGS_REQ message includes information authenticating the
1122
client plus a request for credentials. The authentication information
1123
consists of the authentication header (KRB_AP_REQ) which includes the
1124
client's previously obtained ticket-granting, renewable, or invalid ticket.
1125
In the ticket-granting ticket and proxy cases, the request may include one
1126
or more of: a list of network addresses, a collection of typed authorization
1127
data to be sealed in the ticket for authorization use by the application
1128
server, or additional tickets (the use of which are described later). The
1129
TGS reply (KRB_TGS_REP) contains the requested credentials, encrypted in the
1130
session key from the ticket-granting ticket or renewable ticket, or if
1131
present, in the sub-session key from the Authenticator (part of the
1132
authentication header). The KRB_ERROR message contains an error code and
1133
1134
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1135
1136
1137
1138
1139
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1140
1141
text explaining what went wrong. The KRB_ERROR message is not encrypted. The
1142
KRB_TGS_REP message contains information which can be used to detect
1143
replays, and to associate it with the message to which it replies. The
1144
KRB_ERROR message also contains information which can be used to associate
1145
it with the message to which it replies, but the lack of encryption in the
1146
KRB_ERROR message precludes the ability to detect replays or fabrications of
1147
such messages.
1148
1149
3.3.1. Generation of KRB_TGS_REQ message
1150
1151
Before sending a request to the ticket-granting service, the client must
1152
determine in which realm the application server is registered[15]. If the
1153
client does not already possess a ticket-granting ticket for the appropriate
1154
realm, then one must be obtained. This is first attempted by requesting a
1155
ticket-granting ticket for the destination realm from a Kerberos server for
1156
which the client does posess a ticket-granting ticket (using the KRB_TGS_REQ
1157
message recursively). The Kerberos server may return a TGT for the desired
1158
realm in which case one can proceed. Alternatively, the Kerberos server may
1159
return a TGT for a realm which is 'closer' to the desired realm (further
1160
along the standard hierarchical path), in which case this step must be
1161
repeated with a Kerberos server in the realm specified in the returned TGT.
1162
If neither are returned, then the request must be retried with a Kerberos
1163
server for a realm higher in the hierarchy. This request will itself require
1164
a ticket-granting ticket for the higher realm which must be obtained by
1165
recursively applying these directions.
1166
1167
Once the client obtains a ticket-granting ticket for the appropriate realm,
1168
it determines which Kerberos servers serve that realm, and contacts one. The
1169
list might be obtained through a configuration file or network service or it
1170
may be generated from the name of the realm; as long as the secret keys
1171
exchanged by realms are kept secret, only denial of service results from
1172
using a false Kerberos server.
1173
1174
As in the AS exchange, the client may specify a number of options in the
1175
KRB_TGS_REQ message. The client prepares the KRB_TGS_REQ message, providing
1176
an authentication header as an element of the padata field, and including
1177
the same fields as used in the KRB_AS_REQ message along with several
1178
optional fields: the enc-authorization-data field for application server use
1179
and additional tickets required by some options.
1180
1181
In preparing the authentication header, the client can select a sub-session
1182
key under which the response from the Kerberos server will be encrypted[16].
1183
If the sub-session key is not specified, the session key from the
1184
ticket-granting ticket will be used. If the enc-authorization-data is
1185
present, it must be encrypted in the sub-session key, if present, from the
1186
authenticator portion of the authentication header, or if not present, using
1187
the session key from the ticket-granting ticket.
1188
1189
Once prepared, the message is sent to a Kerberos server for the destination
1190
realm. See section A.5 for pseudocode.
1191
1192
3.3.2. Receipt of KRB_TGS_REQ message
1193
1194
The KRB_TGS_REQ message is processed in a manner similar to the KRB_AS_REQ
1195
message, but there are many additional checks to be performed. First, the
1196
Kerberos server must determine which server the accompanying ticket is for
1197
and it must select the appropriate key to decrypt it. For a normal
1198
KRB_TGS_REQ message, it will be for the ticket granting service, and the
1199
TGS's key will be used. If the TGT was issued by another realm, then the
1200
1201
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1202
1203
1204
1205
1206
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1207
1208
appropriate inter-realm key must be used. If the accompanying ticket is not
1209
a ticket granting ticket for the current realm, but is for an application
1210
server in the current realm, the RENEW, VALIDATE, or PROXY options are
1211
specified in the request, and the server for which a ticket is requested is
1212
the server named in the accompanying ticket, then the KDC will decrypt the
1213
ticket in the authentication header using the key of the server for which it
1214
was issued. If no ticket can be found in the padata field, the
1215
KDC_ERR_PADATA_TYPE_NOSUPP error is returned.
1216
1217
Once the accompanying ticket has been decrypted, the user-supplied checksum
1218
in the Authenticator must be verified against the contents of the request,
1219
and the message rejected if the checksums do not match (with an error code
1220
of KRB_AP_ERR_MODIFIED) or if the checksum is not keyed or not
1221
collision-proof (with an error code of KRB_AP_ERR_INAPP_CKSUM). If the
1222
checksum type is not supported, the KDC_ERR_SUMTYPE_NOSUPP error is
1223
returned. If the authorization-data are present, they are decrypted using
1224
the sub-session key from the Authenticator.
1225
1226
If any of the decryptions indicate failed integrity checks, the
1227
KRB_AP_ERR_BAD_INTEGRITY error is returned.
1228
1229
3.3.3. Generation of KRB_TGS_REP message
1230
1231
The KRB_TGS_REP message shares its format with the KRB_AS_REP (KRB_KDC_REP),
1232
but with its type field set to KRB_TGS_REP. The detailed specification is in
1233
section 5.4.2.
1234
1235
The response will include a ticket for the requested server. The Kerberos
1236
database is queried to retrieve the record for the requested server
1237
(including the key with which the ticket will be encrypted). If the request
1238
is for a ticket granting ticket for a remote realm, and if no key is shared
1239
with the requested realm, then the Kerberos server will select the realm
1240
"closest" to the requested realm with which it does share a key, and use
1241
that realm instead. This is the only case where the response from the KDC
1242
will be for a different server than that requested by the client.
1243
1244
By default, the address field, the client's name and realm, the list of
1245
transited realms, the time of initial authentication, the expiration time,
1246
and the authorization data of the newly-issued ticket will be copied from
1247
the ticket-granting ticket (TGT) or renewable ticket. If the transited field
1248
needs to be updated, but the transited type is not supported, the
1249
KDC_ERR_TRTYPE_NOSUPP error is returned.
1250
1251
If the request specifies an endtime, then the endtime of the new ticket is
1252
set to the minimum of (a) that request, (b) the endtime from the TGT, and
1253
(c) the starttime of the TGT plus the minimum of the maximum life for the
1254
application server and the maximum life for the local realm (the maximum
1255
life for the requesting principal was already applied when the TGT was
1256
issued). If the new ticket is to be a renewal, then the endtime above is
1257
replaced by the minimum of (a) the value of the renew_till field of the
1258
ticket and (b) the starttime for the new ticket plus the life
1259
(endtime-starttime) of the old ticket.
1260
1261
If the FORWARDED option has been requested, then the resulting ticket will
1262
contain the addresses specified by the client. This option will only be
1263
honored if the FORWARDABLE flag is set in the TGT. The PROXY option is
1264
similar; the resulting ticket will contain the addresses specified by the
1265
client. It will be honored only if the PROXIABLE flag in the TGT is set. The
1266
PROXY option will not be honored on requests for additional ticket-granting
1267
1268
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1269
1270
1271
1272
1273
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1274
1275
tickets.
1276
1277
If the requested start time is absent, indicates a time in the past, or is
1278
within the window of acceptable clock skew for the KDC and the POSTDATE
1279
option has not been specified, then the start time of the ticket is set to
1280
the authentication server's current time. If it indicates a time in the
1281
future beyond the acceptable clock skew, but the POSTDATED option has not
1282
been specified or the MAY-POSTDATE flag is not set in the TGT, then the
1283
error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the ticket-granting
1284
ticket has the MAY-POSTDATE flag set, then the resulting ticket will be
1285
postdated and the requested starttime is checked against the policy of the
1286
local realm. If acceptable, the ticket's start time is set as requested, and
1287
the INVALID flag is set. The postdated ticket must be validated before use
1288
by presenting it to the KDC after the starttime has been reached. However,
1289
in no case may the starttime, endtime, or renew-till time of a newly-issued
1290
postdated ticket extend beyond the renew-till time of the ticket-granting
1291
ticket.
1292
1293
If the ENC-TKT-IN-SKEY option has been specified and an additional ticket
1294
has been included in the request, the KDC will decrypt the additional ticket
1295
using the key for the server to which the additional ticket was issued and
1296
verify that it is a ticket-granting ticket. If the name of the requested
1297
server is missing from the request, the name of the client in the additional
1298
ticket will be used. Otherwise the name of the requested server will be
1299
compared to the name of the client in the additional ticket and if
1300
different, the request will be rejected. If the request succeeds, the
1301
session key from the additional ticket will be used to encrypt the new
1302
ticket that is issued instead of using the key of the server for which the
1303
new ticket will be used[17].
1304
1305
If the name of the server in the ticket that is presented to the KDC as part
1306
of the authentication header is not that of the ticket-granting server
1307
itself, the server is registered in the realm of the KDC, and the RENEW
1308
option is requested, then the KDC will verify that the RENEWABLE flag is set
1309
in the ticket, that the INVALID flag is not set in the ticket, and that the
1310
renew_till time is still in the future. If the VALIDATE option is rqeuested,
1311
the KDC will check that the starttime has passed and the INVALID flag is
1312
set. If the PROXY option is requested, then the KDC will check that the
1313
PROXIABLE flag is set in the ticket. If the tests succeed, and the ticket
1314
passes the hotlist check described in the next paragraph, the KDC will issue
1315
the appropriate new ticket.
1316
1317
3.3.3.1. Checking for revoked tickets
1318
1319
Whenever a request is made to the ticket-granting server, the presented
1320
ticket(s) is(are) checked against a hot-list of tickets which have been
1321
canceled. This hot-list might be implemented by storing a range of issue
1322
timestamps for 'suspect tickets'; if a presented ticket had an authtime in
1323
that range, it would be rejected. In this way, a stolen ticket-granting
1324
ticket or renewable ticket cannot be used to gain additional tickets
1325
(renewals or otherwise) once the theft has been reported. Any normal ticket
1326
obtained before it was reported stolen will still be valid (because they
1327
require no interaction with the KDC), but only until their normal expiration
1328
time.
1329
1330
The ciphertext part of the response in the KRB_TGS_REP message is encrypted
1331
in the sub-session key from the Authenticator, if present, or the session
1332
key key from the ticket-granting ticket. It is not encrypted using the
1333
client's secret key. Furthermore, the client's key's expiration date and the
1334
1335
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1336
1337
1338
1339
1340
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1341
1342
key version number fields are left out since these values are stored along
1343
with the client's database record, and that record is not needed to satisfy
1344
a request based on a ticket-granting ticket. See section A.6 for pseudocode.
1345
1346
3.3.3.2. Encoding the transited field
1347
1348
If the identity of the server in the TGT that is presented to the KDC as
1349
part of the authentication header is that of the ticket-granting service,
1350
but the TGT was issued from another realm, the KDC will look up the
1351
inter-realm key shared with that realm and use that key to decrypt the
1352
ticket. If the ticket is valid, then the KDC will honor the request, subject
1353
to the constraints outlined above in the section describing the AS exchange.
1354
The realm part of the client's identity will be taken from the
1355
ticket-granting ticket. The name of the realm that issued the
1356
ticket-granting ticket will be added to the transited field of the ticket to
1357
be issued. This is accomplished by reading the transited field from the
1358
ticket-granting ticket (which is treated as an unordered set of realm
1359
names), adding the new realm to the set, then constructing and writing out
1360
its encoded (shorthand) form (this may involve a rearrangement of the
1361
existing encoding).
1362
1363
Note that the ticket-granting service does not add the name of its own
1364
realm. Instead, its responsibility is to add the name of the previous realm.
1365
This prevents a malicious Kerberos server from intentionally leaving out its
1366
own name (it could, however, omit other realms' names).
1367
1368
The names of neither the local realm nor the principal's realm are to be
1369
included in the transited field. They appear elsewhere in the ticket and
1370
both are known to have taken part in authenticating the principal. Since the
1371
endpoints are not included, both local and single-hop inter-realm
1372
authentication result in a transited field that is empty.
1373
1374
Because the name of each realm transited is added to this field, it might
1375
potentially be very long. To decrease the length of this field, its contents
1376
are encoded. The initially supported encoding is optimized for the normal
1377
case of inter-realm communication: a hierarchical arrangement of realms
1378
using either domain or X.500 style realm names. This encoding (called
1379
DOMAIN-X500-COMPRESS) is now described.
1380
1381
Realm names in the transited field are separated by a ",". The ",", "\",
1382
trailing "."s, and leading spaces (" ") are special characters, and if they
1383
are part of a realm name, they must be quoted in the transited field by
1384
preced- ing them with a "\".
1385
1386
A realm name ending with a "." is interpreted as being prepended to the
1387
previous realm. For example, we can encode traversal of EDU, MIT.EDU,
1388
ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:
1389
1390
"EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
1391
1392
Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points, that they
1393
would not be included in this field, and we would have:
1394
1395
"EDU,MIT.,WASHINGTON.EDU"
1396
1397
A realm name beginning with a "/" is interpreted as being appended to the
1398
previous realm[18]. If it is to stand by itself, then it should be preceded
1399
by a space (" "). For example, we can encode traversal of /COM/HP/APOLLO,
1400
/COM/HP, /COM, and /COM/DEC as:
1401
1402
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1403
1404
1405
1406
1407
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1408
1409
1410
"/COM,/HP,/APOLLO, /COM/DEC".
1411
1412
Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints, they
1413
they would not be included in this field, and we would have:
1414
1415
"/COM,/HP"
1416
1417
A null subfield preceding or following a "," indicates that all realms
1418
between the previous realm and the next realm have been traversed[19]. Thus,
1419
"," means that all realms along the path between the client and the server
1420
have been traversed. ",EDU, /COM," means that that all realms from the
1421
client's realm up to EDU (in a domain style hierarchy) have been traversed,
1422
and that everything from /COM down to the server's realm in an X.500 style
1423
has also been traversed. This could occur if the EDU realm in one hierarchy
1424
shares an inter-realm key directly with the /COM realm in another hierarchy.
1425
1426
3.3.4. Receipt of KRB_TGS_REP message
1427
1428
When the KRB_TGS_REP is received by the client, it is processed in the same
1429
manner as the KRB_AS_REP processing described above. The primary difference
1430
is that the ciphertext part of the response must be decrypted using the
1431
session key from the ticket-granting ticket rather than the client's secret
1432
key. See section A.7 for pseudocode.
1433
1434
3.4. The KRB_SAFE Exchange
1435
1436
The KRB_SAFE message may be used by clients requiring the ability to detect
1437
modifications of messages they exchange. It achieves this by including a
1438
keyed collision-proof checksum of the user data and some control
1439
information. The checksum is keyed with an encryption key (usually the last
1440
key negotiated via subkeys, or the session key if no negotiation has
1441
occured).
1442
1443
3.4.1. Generation of a KRB_SAFE message
1444
1445
When an application wishes to send a KRB_SAFE message, it collects its data
1446
and the appropriate control information and computes a checksum over them.
1447
The checksum algorithm should be a keyed one-way hash function (such as the
1448
RSA- MD5-DES checksum algorithm specified in section 6.4.5, or the DES MAC),
1449
generated using the sub-session key if present, or the session key.
1450
Different algorithms may be selected by changing the checksum type in the
1451
message. Unkeyed or non-collision-proof checksums are not suitable for this
1452
use.
1453
1454
The control information for the KRB_SAFE message includes both a timestamp
1455
and a sequence number. The designer of an application using the KRB_SAFE
1456
message must choose at least one of the two mechanisms. This choice should
1457
be based on the needs of the application protocol.
1458
1459
Sequence numbers are useful when all messages sent will be received by one's
1460
peer. Connection state is presently required to maintain the session key, so
1461
maintaining the next sequence number should not present an additional
1462
problem.
1463
1464
If the application protocol is expected to tolerate lost messages without
1465
them being resent, the use of the timestamp is the appropriate replay
1466
detection mechanism. Using timestamps is also the appropriate mechanism for
1467
multi-cast protocols where all of one's peers share a common sub-session
1468
1469
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1470
1471
1472
1473
1474
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1475
1476
key, but some messages will be sent to a subset of one's peers.
1477
1478
After computing the checksum, the client then transmits the information and
1479
checksum to the recipient in the message format specified in section 5.6.1.
1480
1481
3.4.2. Receipt of KRB_SAFE message
1482
1483
When an application receives a KRB_SAFE message, it verifies it as follows.
1484
If any error occurs, an error code is reported for use by the application.
1485
1486
The message is first checked by verifying that the protocol version and type
1487
fields match the current version and KRB_SAFE, respectively. A mismatch
1488
generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
1489
application verifies that the checksum used is a collision-proof keyed
1490
checksum, and if it is not, a KRB_AP_ERR_INAPP_CKSUM error is generated. If
1491
the sender's address was included in the control information, the recipient
1492
verifies that the operating system's report of the sender's address matches
1493
the sender's address in the message, and (if a recipient address is
1494
specified or the recipient requires an address) that one of the recipient's
1495
addresses appears as the recipient's address in the message. A failed match
1496
for either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp and
1497
usec and/or the sequence number fields are checked. If timestamp and usec
1498
are expected and not present, or they are present but not current, the
1499
KRB_AP_ERR_SKEW error is generated. If the server name, along with the
1500
client name, time and microsecond fields from the Authenticator match any
1501
recently-seen (sent or received[20] ) such tuples, the KRB_AP_ERR_REPEAT
1502
error is generated. If an incorrect sequence number is included, or a
1503
sequence number is expected but not present, the KRB_AP_ERR_BADORDER error
1504
is generated. If neither a time-stamp and usec or a sequence number is
1505
present, a KRB_AP_ERR_MODIFIED error is generated. Finally, the checksum is
1506
computed over the data and control information, and if it doesn't match the
1507
received checksum, a KRB_AP_ERR_MODIFIED error is generated.
1508
1509
If all the checks succeed, the application is assured that the message was
1510
generated by its peer and was not modi- fied in transit.
1511
1512
3.5. The KRB_PRIV Exchange
1513
1514
The KRB_PRIV message may be used by clients requiring confidentiality and
1515
the ability to detect modifications of exchanged messages. It achieves this
1516
by encrypting the messages and adding control information.
1517
1518
3.5.1. Generation of a KRB_PRIV message
1519
1520
When an application wishes to send a KRB_PRIV message, it collects its data
1521
and the appropriate control information (specified in section 5.7.1) and
1522
encrypts them under an encryption key (usually the last key negotiated via
1523
subkeys, or the session key if no negotiation has occured). As part of the
1524
control information, the client must choose to use either a timestamp or a
1525
sequence number (or both); see the discussion in section 3.4.1 for
1526
guidelines on which to use. After the user data and control information are
1527
encrypted, the client transmits the ciphertext and some 'envelope'
1528
information to the recipient.
1529
1530
3.5.2. Receipt of KRB_PRIV message
1531
1532
When an application receives a KRB_PRIV message, it verifies it as follows.
1533
If any error occurs, an error code is reported for use by the application.
1534
1535
1536
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1537
1538
1539
1540
1541
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1542
1543
The message is first checked by verifying that the protocol version and type
1544
fields match the current version and KRB_PRIV, respectively. A mismatch
1545
generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
1546
application then decrypts the ciphertext and processes the resultant
1547
plaintext. If decryption shows the data to have been modified, a
1548
KRB_AP_ERR_BAD_INTEGRITY error is generated. If the sender's address was
1549
included in the control information, the recipient verifies that the
1550
operating system's report of the sender's address matches the sender's
1551
address in the message, and (if a recipient address is specified or the
1552
recipient requires an address) that one of the recipient's addresses appears
1553
as the recipient's address in the message. A failed match for either case
1554
generates a KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the
1555
sequence number fields are checked. If timestamp and usec are expected and
1556
not present, or they are present but not current, the KRB_AP_ERR_SKEW error
1557
is generated. If the server name, along with the client name, time and
1558
microsecond fields from the Authenticator match any recently-seen such
1559
tuples, the KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence
1560
number is included, or a sequence number is expected but not present, the
1561
KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and usec or
1562
a sequence number is present, a KRB_AP_ERR_MODIFIED error is generated.
1563
1564
If all the checks succeed, the application can assume the message was
1565
generated by its peer, and was securely transmitted (without intruders able
1566
to see the unencrypted contents).
1567
1568
3.6. The KRB_CRED Exchange
1569
1570
The KRB_CRED message may be used by clients requiring the ability to send
1571
Kerberos credentials from one host to another. It achieves this by sending
1572
the tickets together with encrypted data containing the session keys and
1573
other information associated with the tickets.
1574
1575
3.6.1. Generation of a KRB_CRED message
1576
1577
When an application wishes to send a KRB_CRED message it first (using the
1578
KRB_TGS exchange) obtains credentials to be sent to the remote host. It then
1579
constructs a KRB_CRED message using the ticket or tickets so obtained,
1580
placing the session key needed to use each ticket in the key field of the
1581
corresponding KrbCredInfo sequence of the encrypted part of the the KRB_CRED
1582
message.
1583
1584
Other information associated with each ticket and obtained during the
1585
KRB_TGS exchange is also placed in the corresponding KrbCredInfo sequence in
1586
the encrypted part of the KRB_CRED message. The current time and, if
1587
specifically required by the application the nonce, s-address, and r-address
1588
fields, are placed in the encrypted part of the KRB_CRED message which is
1589
then encrypted under an encryption key previosuly exchanged in the KRB_AP
1590
exchange (usually the last key negotiated via subkeys, or the session key if
1591
no negotiation has occured).
1592
1593
3.6.2. Receipt of KRB_CRED message
1594
1595
When an application receives a KRB_CRED message, it verifies it. If any
1596
error occurs, an error code is reported for use by the application. The
1597
message is verified by checking that the protocol version and type fields
1598
match the current version and KRB_CRED, respectively. A mismatch generates a
1599
KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then
1600
decrypts the ciphertext and processes the resultant plaintext. If decryption
1601
shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is
1602
1603
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1604
1605
1606
1607
1608
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1609
1610
generated.
1611
1612
If present or required, the recipient verifies that the operating system's
1613
report of the sender's address matches the sender's address in the message,
1614
and that one of the recipient's addresses appears as the recipient's address
1615
in the message. A failed match for either case generates a
1616
KRB_AP_ERR_BADADDR error. The timestamp and usec fields (and the nonce field
1617
if required) are checked next. If the timestamp and usec are not present, or
1618
they are present but not current, the KRB_AP_ERR_SKEW error is generated.
1619
1620
If all the checks succeed, the application stores each of the new tickets in
1621
its ticket cache together with the session key and other information in the
1622
corresponding KrbCredInfo sequence from the encrypted part of the KRB_CRED
1623
message.
1624
1625
4. The Kerberos Database
1626
1627
The Kerberos server must have access to a database containing the principal
1628
identifiers and secret keys of principals to be authenticated[21].
1629
1630
4.1. Database contents
1631
1632
A database entry should contain at least the following fields:
1633
1634
Field Value
1635
1636
name Principal's identifier
1637
key Principal's secret key
1638
p_kvno Principal's key version
1639
max_life Maximum lifetime for Tickets
1640
max_renewable_life Maximum total lifetime for renewable Tickets
1641
1642
The name field is an encoding of the principal's identifier. The key field
1643
contains an encryption key. This key is the principal's secret key. (The key
1644
can be encrypted before storage under a Kerberos "master key" to protect it
1645
in case the database is compromised but the master key is not. In that case,
1646
an extra field must be added to indicate the master key version used, see
1647
below.) The p_kvno field is the key version number of the principal's secret
1648
key. The max_life field contains the maximum allowable lifetime (endtime -
1649
starttime) for any Ticket issued for this principal. The max_renewable_life
1650
field contains the maximum allowable total lifetime for any renewable Ticket
1651
issued for this principal. (See section 3.1 for a description of how these
1652
lifetimes are used in determining the lifetime of a given Ticket.)
1653
1654
A server may provide KDC service to several realms, as long as the database
1655
representation provides a mechanism to distinguish between principal records
1656
with identifiers which differ only in the realm name.
1657
1658
When an application server's key changes, if the change is routine (i.e. not
1659
the result of disclosure of the old key), the old key should be retained by
1660
the server until all tickets that had been issued using that key have
1661
expired. Because of this, it is possible for several keys to be active for a
1662
single principal. Ciphertext encrypted in a principal's key is always tagged
1663
with the version of the key that was used for encryption, to help the
1664
recipient find the proper key for decryption.
1665
1666
When more than one key is active for a particular principal, the principal
1667
will have more than one record in the Kerberos database. The keys and key
1668
version numbers will differ between the records (the rest of the fields may
1669
1670
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1671
1672
1673
1674
1675
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1676
1677
or may not be the same). Whenever Kerberos issues a ticket, or responds to a
1678
request for initial authentication, the most recent key (known by the
1679
Kerberos server) will be used for encryption. This is the key with the
1680
highest key version number.
1681
1682
4.2. Additional fields
1683
1684
Project Athena's KDC implementation uses additional fields in its database:
1685
1686
Field Value
1687
1688
K_kvno Kerberos' key version
1689
expiration Expiration date for entry
1690
attributes Bit field of attributes
1691
mod_date Timestamp of last modification
1692
mod_name Modifying principal's identifier
1693
1694
The K_kvno field indicates the key version of the Kerberos master key under
1695
which the principal's secret key is encrypted.
1696
1697
After an entry's expiration date has passed, the KDC will return an error to
1698
any client attempting to gain tickets as or for the principal. (A database
1699
may want to maintain two expiration dates: one for the principal, and one
1700
for the principal's current key. This allows password aging to work
1701
independently of the principal's expiration date. However, due to the
1702
limited space in the responses, the KDC must combine the key expiration and
1703
principal expiration date into a single value called 'key_exp', which is
1704
used as a hint to the user to take administrative action.)
1705
1706
The attributes field is a bitfield used to govern the operations involving
1707
the principal. This field might be useful in conjunction with user
1708
registration procedures, for site-specific policy implementations (Project
1709
Athena currently uses it for their user registration process controlled by
1710
the system-wide database service, Moira [LGDSR87]), to identify whether a
1711
principal can play the role of a client or server or both, to note whether a
1712
server is appropriate trusted to recieve credentials delegated by a client,
1713
or to identify the 'string to key' conversion algorithm used for a
1714
principal's key[22]. Other bits are used to indicate that certain ticket
1715
options should not be allowed in tickets encrypted under a principal's key
1716
(one bit each): Disallow issuing postdated tickets, disallow issuing
1717
forwardable tickets, disallow issuing tickets based on TGT authentication,
1718
disallow issuing renewable tickets, disallow issuing proxiable tickets, and
1719
disallow issuing tickets for which the principal is the server.
1720
1721
The mod_date field contains the time of last modification of the entry, and
1722
the mod_name field contains the name of the principal which last modified
1723
the entry.
1724
1725
4.3. Frequently Changing Fields
1726
1727
Some KDC implementations may wish to maintain the last time that a request
1728
was made by a particular principal. Information that might be maintained
1729
includes the time of the last request, the time of the last request for a
1730
ticket-granting ticket, the time of the last use of a ticket-granting
1731
ticket, or other times. This information can then be returned to the user in
1732
the last-req field (see section 5.2).
1733
1734
Other frequently changing information that can be maintained is the latest
1735
expiration time for any tickets that have been issued using each key. This
1736
1737
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1738
1739
1740
1741
1742
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1743
1744
field would be used to indicate how long old keys must remain valid to allow
1745
the continued use of outstanding tickets.
1746
1747
4.4. Site Constants
1748
1749
The KDC implementation should have the following configurable constants or
1750
options, to allow an administrator to make and enforce policy decisions:
1751
1752
* The minimum supported lifetime (used to determine whether the
1753
KDC_ERR_NEVER_VALID error should be returned). This constant should
1754
reflect reasonable expectations of round-trip time to the KDC,
1755
encryption/decryption time, and processing time by the client and
1756
target server, and it should allow for a minimum 'useful' lifetime.
1757
* The maximum allowable total (renewable) lifetime of a ticket
1758
(renew_till - starttime).
1759
* The maximum allowable lifetime of a ticket (endtime - starttime).
1760
* Whether to allow the issue of tickets with empty address fields
1761
(including the ability to specify that such tickets may only be issued
1762
if the request specifies some authorization_data).
1763
* Whether proxiable, forwardable, renewable or post-datable tickets are
1764
to be issued.
1765
1766
5. Message Specifications
1767
1768
The following sections describe the exact contents and encoding of protocol
1769
messages and objects. The ASN.1 base definitions are presented in the first
1770
subsection. The remaining subsections specify the protocol objects (tickets
1771
and authenticators) and messages. Specification of encryption and checksum
1772
techniques, and the fields related to them, appear in section 6.
1773
1774
Optional field in ASN.1 sequences
1775
1776
For optional integer value and date fields in ASN.1 sequences where a
1777
default value has been specified, certain default values will not be allowed
1778
in the encoding because these values will always be represented through
1779
defaulting by the absence of the optional field. For example, one will not
1780
send a microsecond zero value because one must make sure that there is only
1781
one way to encode this value.
1782
1783
Additional fields in ASN.1 sequences
1784
1785
Implementations receiving Kerberos messages with additional fields present
1786
in ASN.1 sequences should carry the those fields through, unmodified, when
1787
the message is forwarded. Implementations should not drop such fields if the
1788
sequence is reencoded.
1789
1790
5.1. ASN.1 Distinguished Encoding Representation
1791
1792
All uses of ASN.1 in Kerberos shall use the Distinguished Encoding
1793
Representation of the data elements as described in the X.509 specification,
1794
section 8.7 [X509-88].
1795
1796
5.3. ASN.1 Base Definitions
1797
1798
The following ASN.1 base definitions are used in the rest of this section.
1799
Note that since the underscore character (_) is not permitted in ASN.1
1800
names, the hyphen (-) is used in its place for the purposes of ASN.1 names.
1801
1802
Realm ::= GeneralString
1803
1804
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1805
1806
1807
1808
1809
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1810
1811
PrincipalName ::= SEQUENCE {
1812
name-type[0] INTEGER,
1813
name-string[1] SEQUENCE OF GeneralString
1814
}
1815
1816
Kerberos realms are encoded as GeneralStrings. Realms shall not contain a
1817
character with the code 0 (the ASCII NUL). Most realms will usually consist
1818
of several components separated by periods (.), in the style of Internet
1819
Domain Names, or separated by slashes (/) in the style of X.500 names.
1820
Acceptable forms for realm names are specified in section 7. A PrincipalName
1821
is a typed sequence of components consisting of the following sub-fields:
1822
1823
name-type
1824
This field specifies the type of name that follows. Pre-defined values
1825
for this field are specified in section 7.2. The name-type should be
1826
treated as a hint. Ignoring the name type, no two names can be the same
1827
(i.e. at least one of the components, or the realm, must be different).
1828
This constraint may be eliminated in the future.
1829
name-string
1830
This field encodes a sequence of components that form a name, each
1831
component encoded as a GeneralString. Taken together, a PrincipalName
1832
and a Realm form a principal identifier. Most PrincipalNames will have
1833
only a few components (typically one or two).
1834
1835
KerberosTime ::= GeneralizedTime
1836
-- Specifying UTC time zone (Z)
1837
1838
The timestamps used in Kerberos are encoded as GeneralizedTimes. An encoding
1839
shall specify the UTC time zone (Z) and shall not include any fractional
1840
portions of the seconds. It further shall not include any separators.
1841
Example: The only valid format for UTC time 6 minutes, 27 seconds after 9 pm
1842
on 6 November 1985 is 19851106210627Z.
1843
1844
HostAddress ::= SEQUENCE {
1845
addr-type[0] INTEGER,
1846
address[1] OCTET STRING
1847
}
1848
1849
HostAddresses ::= SEQUENCE OF HostAddress
1850
1851
The host adddress encodings consists of two fields:
1852
1853
addr-type
1854
This field specifies the type of address that follows. Pre-defined
1855
values for this field are specified in section 8.1.
1856
address
1857
This field encodes a single address of type addr-type.
1858
1859
The two forms differ slightly. HostAddress contains exactly one address;
1860
HostAddresses contains a sequence of possibly many addresses.
1861
1862
AuthorizationData ::= SEQUENCE OF SEQUENCE {
1863
ad-type[0] INTEGER,
1864
ad-data[1] OCTET STRING
1865
}
1866
1867
ad-data
1868
This field contains authorization data to be interpreted according to
1869
the value of the corresponding ad-type field.
1870
1871
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1872
1873
1874
1875
1876
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1877
1878
ad-type
1879
This field specifies the format for the ad-data subfield. All negative
1880
values are reserved for local use. Non-negative values are reserved for
1881
registered use.
1882
1883
Each sequence of type and data is refered to as an authorization element.
1884
Elements may be application specific, however, there is a common set of
1885
recursive elements that should be understood by all implementations. These
1886
elements contain other elements embedded within them, and the interpretation
1887
of the encapsulating element determines which of the embedded elements must
1888
be interpreted, and which may be ignored. Definitions for these common
1889
elements may be found in Appendix B.
1890
1891
TicketExtensions ::= SEQUENCE OF SEQUENCE {
1892
te-type[0] INTEGER,
1893
te-data[1] OCTET STRING
1894
}
1895
1896
1897
1898
te-data
1899
This field contains opaque data that must be caried with the ticket to
1900
support extensions to the Kerberos protocol including but not limited
1901
to some forms of inter-realm key exchange and plaintext authorization
1902
data. See appendix C for some common uses of this field.
1903
te-type
1904
This field specifies the format for the te-data subfield. All negative
1905
values are reserved for local use. Non-negative values are reserved for
1906
registered use.
1907
1908
APOptions ::= BIT STRING
1909
-- reserved(0),
1910
-- use-session-key(1),
1911
-- mutual-required(2)
1912
1913
TicketFlags ::= BIT STRING
1914
-- reserved(0),
1915
-- forwardable(1),
1916
-- forwarded(2),
1917
-- proxiable(3),
1918
-- proxy(4),
1919
-- may-postdate(5),
1920
-- postdated(6),
1921
-- invalid(7),
1922
-- renewable(8),
1923
-- initial(9),
1924
-- pre-authent(10),
1925
-- hw-authent(11),
1926
-- transited-policy-checked(12),
1927
-- ok-as-delegate(13)
1928
1929
KDCOptions ::= BIT STRING
1930
-- reserved(0),
1931
-- forwardable(1),
1932
-- forwarded(2),
1933
-- proxiable(3),
1934
-- proxy(4),
1935
-- allow-postdate(5),
1936
-- postdated(6),
1937
1938
Neuman, Ts'o, Kohl Expires: 10 September, 2000
1939
1940
1941
1942
1943
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
1944
1945
-- unused7(7),
1946
-- renewable(8),
1947
-- unused9(9),
1948
-- unused10(10),
1949
-- unused11(11),
1950
-- unused12(12),
1951
-- unused13(13),
1952
-- disable-transited-check(26),
1953
-- renewable-ok(27),
1954
-- enc-tkt-in-skey(28),
1955
-- renew(30),
1956
-- validate(31)
1957
1958
ASN.1 Bit strings have a length and a value. When used in Kerberos for the
1959
APOptions, TicketFlags, and KDCOptions, the length of the bit string on
1960
generated values should be the smallest number of bits needed to include the
1961
highest order bit that is set (1), but in no case less than 32 bits. The
1962
ASN.1 representation of the bit strings uses unnamed bits, with the meaning
1963
of the individual bits defined by the comments in the specification above.
1964
Implementations should accept values of bit strings of any length and treat
1965
the value of flags corresponding to bits beyond the end of the bit string as
1966
if the bit were reset (0). Comparison of bit strings of different length
1967
should treat the smaller string as if it were padded with zeros beyond the
1968
high order bits to the length of the longer string[23].
1969
1970
LastReq ::= SEQUENCE OF SEQUENCE {
1971
lr-type[0] INTEGER,
1972
lr-value[1] KerberosTime
1973
}
1974
1975
lr-type
1976
This field indicates how the following lr-value field is to be
1977
interpreted. Negative values indicate that the information pertains
1978
only to the responding server. Non-negative values pertain to all
1979
servers for the realm. If the lr-type field is zero (0), then no
1980
information is conveyed by the lr-value subfield. If the absolute value
1981
of the lr-type field is one (1), then the lr-value subfield is the time
1982
of last initial request for a TGT. If it is two (2), then the lr-value
1983
subfield is the time of last initial request. If it is three (3), then
1984
the lr-value subfield is the time of issue for the newest
1985
ticket-granting ticket used. If it is four (4), then the lr-value
1986
subfield is the time of the last renewal. If it is five (5), then the
1987
lr-value subfield is the time of last request (of any type). If it is
1988
(6), then the lr-value subfield is the time when the password will
1989
expire.
1990
lr-value
1991
This field contains the time of the last request. the time must be
1992
interpreted according to the contents of the accompanying lr-type
1993
subfield.
1994
1995
See section 6 for the definitions of Checksum, ChecksumType, EncryptedData,
1996
EncryptionKey, EncryptionType, and KeyType.
1997
1998
5.3. Tickets and Authenticators
1999
2000
This section describes the format and encryption parameters for tickets and
2001
authenticators. When a ticket or authenticator is included in a protocol
2002
message it is treated as an opaque object.
2003
2004
2005
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2006
2007
2008
2009
2010
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2011
2012
5.3.1. Tickets
2013
2014
A ticket is a record that helps a client authenticate to a service. A Ticket
2015
contains the following information:
2016
2017
Ticket ::= [APPLICATION 1] SEQUENCE {
2018
tkt-vno[0] INTEGER,
2019
realm[1] Realm,
2020
sname[2] PrincipalName,
2021
enc-part[3] EncryptedData,
2022
extensions[4] TicketExtensions OPTIONAL
2023
}
2024
2025
-- Encrypted part of ticket
2026
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
2027
flags[0] TicketFlags,
2028
key[1] EncryptionKey,
2029
crealm[2] Realm,
2030
cname[3] PrincipalName,
2031
transited[4] TransitedEncoding,
2032
authtime[5] KerberosTime,
2033
starttime[6] KerberosTime OPTIONAL,
2034
endtime[7] KerberosTime,
2035
renew-till[8] KerberosTime OPTIONAL,
2036
caddr[9] HostAddresses OPTIONAL,
2037
authorization-data[10] AuthorizationData OPTIONAL
2038
}
2039
-- encoded Transited field
2040
TransitedEncoding ::= SEQUENCE {
2041
tr-type[0] INTEGER, -- must be registered
2042
contents[1] OCTET STRING
2043
}
2044
2045
The encoding of EncTicketPart is encrypted in the key shared by Kerberos and
2046
the end server (the server's secret key). See section 6 for the format of
2047
the ciphertext.
2048
2049
tkt-vno
2050
This field specifies the version number for the ticket format. This
2051
document describes version number 5.
2052
realm
2053
This field specifies the realm that issued a ticket. It also serves to
2054
identify the realm part of the server's principal identifier. Since a
2055
Kerberos server can only issue tickets for servers within its realm,
2056
the two will always be identical.
2057
sname
2058
This field specifies all components of the name part of the server's
2059
identity, including those parts that identify a specific instance of a
2060
service.
2061
enc-part
2062
This field holds the encrypted encoding of the EncTicketPart sequence.
2063
extensions
2064
This optional field contains a sequence of extentions that may be used
2065
to carry information that must be carried with the ticket to support
2066
several extensions, including but not limited to plaintext
2067
authorization data, tokens for exchanging inter-realm keys, and other
2068
information that must be associated with a ticket for use by the
2069
application server. See Appendix C for definitions of some common
2070
extensions.
2071
2072
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2073
2074
2075
2076
2077
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2078
2079
2080
Note that some older versions of Kerberos did not support this field.
2081
Because this is an optional field it will not break older clients, but
2082
older clients might strip this field from the ticket before sending it
2083
to the application server. This limits the usefulness of this ticket
2084
field to environments where the ticket will not be parsed and
2085
reconstructed by these older Kerberos clients.
2086
2087
If it is known that the client will strip this field from the ticket,
2088
as an interim measure the KDC may append this field to the end of the
2089
enc-part of the ticket and append a traler indicating the lenght of the
2090
appended extensions field. (this paragraph is open for discussion,
2091
including the form of the traler).
2092
flags
2093
This field indicates which of various options were used or requested
2094
when the ticket was issued. It is a bit-field, where the selected
2095
options are indicated by the bit being set (1), and the unselected
2096
options and reserved fields being reset (0). Bit 0 is the most
2097
significant bit. The encoding of the bits is specified in section 5.2.
2098
The flags are described in more detail above in section 2. The meanings
2099
of the flags are:
2100
2101
Bit(s) Name Description
2102
2103
0 RESERVED
2104
Reserved for future expansion of this
2105
field.
2106
2107
1 FORWARDABLE
2108
The FORWARDABLE flag is normally only
2109
interpreted by the TGS, and can be
2110
ignored by end servers. When set, this
2111
flag tells the ticket-granting server
2112
that it is OK to issue a new ticket-
2113
granting ticket with a different network
2114
address based on the presented ticket.
2115
2116
2 FORWARDED
2117
When set, this flag indicates that the
2118
ticket has either been forwarded or was
2119
issued based on authentication involving
2120
a forwarded ticket-granting ticket.
2121
2122
3 PROXIABLE
2123
The PROXIABLE flag is normally only
2124
interpreted by the TGS, and can be
2125
ignored by end servers. The PROXIABLE
2126
flag has an interpretation identical to
2127
that of the FORWARDABLE flag, except
2128
that the PROXIABLE flag tells the
2129
ticket-granting server that only non-
2130
ticket-granting tickets may be issued
2131
with different network addresses.
2132
2133
4 PROXY
2134
When set, this flag indicates that a
2135
ticket is a proxy.
2136
2137
5 MAY-POSTDATE
2138
2139
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2140
2141
2142
2143
2144
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2145
2146
The MAY-POSTDATE flag is normally only
2147
interpreted by the TGS, and can be
2148
ignored by end servers. This flag tells
2149
the ticket-granting server that a post-
2150
dated ticket may be issued based on this
2151
ticket-granting ticket.
2152
2153
6 POSTDATED
2154
This flag indicates that this ticket has
2155
been postdated. The end-service can
2156
check the authtime field to see when the
2157
original authentication occurred.
2158
2159
7 INVALID
2160
This flag indicates that a ticket is
2161
invalid, and it must be validated by the
2162
KDC before use. Application servers
2163
must reject tickets which have this flag
2164
set.
2165
2166
8 RENEWABLE
2167
The RENEWABLE flag is normally only
2168
interpreted by the TGS, and can usually
2169
be ignored by end servers (some particu-
2170
larly careful servers may wish to disal-
2171
low renewable tickets). A renewable
2172
ticket can be used to obtain a replace-
2173
ment ticket that expires at a later
2174
date.
2175
2176
9 INITIAL
2177
This flag indicates that this ticket was
2178
issued using the AS protocol, and not
2179
issued based on a ticket-granting
2180
ticket.
2181
2182
10 PRE-AUTHENT
2183
This flag indicates that during initial
2184
authentication, the client was authenti-
2185
cated by the KDC before a ticket was
2186
issued. The strength of the pre-
2187
authentication method is not indicated,
2188
but is acceptable to the KDC.
2189
2190
11 HW-AUTHENT
2191
This flag indicates that the protocol
2192
employed for initial authentication
2193
required the use of hardware expected to
2194
be possessed solely by the named client.
2195
The hardware authentication method is
2196
selected by the KDC and the strength of
2197
the method is not indicated.
2198
2199
12 TRANSITED This flag indicates that the KDC for the
2200
POLICY-CHECKED realm has checked the transited field
2201
against a realm defined policy for
2202
trusted certifiers. If this flag is
2203
reset (0), then the application server
2204
must check the transited field itself,
2205
2206
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2207
2208
2209
2210
2211
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2212
2213
and if unable to do so it must reject
2214
the authentication. If the flag is set
2215
(1) then the application server may skip
2216
its own validation of the transited
2217
field, relying on the validation
2218
performed by the KDC. At its option the
2219
application server may still apply its
2220
own validation based on a separate
2221
policy for acceptance.
2222
2223
13 OK-AS-DELEGATE This flag indicates that the server (not
2224
the client) specified in the ticket has
2225
been determined by policy of the realm
2226
to be a suitable recipient of
2227
delegation. A client can use the
2228
presence of this flag to help it make a
2229
decision whether to delegate credentials
2230
(either grant a proxy or a forwarded
2231
ticket granting ticket) to this server.
2232
The client is free to ignore the value
2233
of this flag. When setting this flag,
2234
an administrator should consider the
2235
Security and placement of the server on
2236
which the service will run, as well as
2237
whether the service requires the use of
2238
delegated credentials.
2239
2240
14 ANONYMOUS
2241
This flag indicates that the principal
2242
named in the ticket is a generic princi-
2243
pal for the realm and does not identify
2244
the individual using the ticket. The
2245
purpose of the ticket is only to
2246
securely distribute a session key, and
2247
not to identify the user. Subsequent
2248
requests using the same ticket and ses-
2249
sion may be considered as originating
2250
from the same user, but requests with
2251
the same username but a different ticket
2252
are likely to originate from different
2253
users.
2254
2255
15-31 RESERVED
2256
Reserved for future use.
2257
2258
key
2259
This field exists in the ticket and the KDC response and is used to
2260
pass the session key from Kerberos to the application server and the
2261
client. The field's encoding is described in section 6.2.
2262
crealm
2263
This field contains the name of the realm in which the client is
2264
registered and in which initial authentication took place.
2265
cname
2266
This field contains the name part of the client's principal identifier.
2267
transited
2268
This field lists the names of the Kerberos realms that took part in
2269
authenticating the user to whom this ticket was issued. It does not
2270
specify the order in which the realms were transited. See section
2271
3.3.3.2 for details on how this field encodes the traversed realms.
2272
2273
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2274
2275
2276
2277
2278
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2279
2280
When the names of CA's are to be embedded inthe transited field (as
2281
specified for some extentions to the protocol), the X.500 names of the
2282
CA's should be mapped into items in the transited field using the
2283
mapping defined by RFC2253.
2284
authtime
2285
This field indicates the time of initial authentication for the named
2286
principal. It is the time of issue for the original ticket on which
2287
this ticket is based. It is included in the ticket to provide
2288
additional information to the end service, and to provide the necessary
2289
information for implementation of a `hot list' service at the KDC. An
2290
end service that is particularly paranoid could refuse to accept
2291
tickets for which the initial authentication occurred "too far" in the
2292
past. This field is also returned as part of the response from the KDC.
2293
When returned as part of the response to initial authentication
2294
(KRB_AS_REP), this is the current time on the Kerberos server[24].
2295
starttime
2296
This field in the ticket specifies the time after which the ticket is
2297
valid. Together with endtime, this field specifies the life of the
2298
ticket. If it is absent from the ticket, its value should be treated as
2299
that of the authtime field.
2300
endtime
2301
This field contains the time after which the ticket will not be honored
2302
(its expiration time). Note that individual services may place their
2303
own limits on the life of a ticket and may reject tickets which have
2304
not yet expired. As such, this is really an upper bound on the
2305
expiration time for the ticket.
2306
renew-till
2307
This field is only present in tickets that have the RENEWABLE flag set
2308
in the flags field. It indicates the maximum endtime that may be
2309
included in a renewal. It can be thought of as the absolute expiration
2310
time for the ticket, including all renewals.
2311
caddr
2312
This field in a ticket contains zero (if omitted) or more (if present)
2313
host addresses. These are the addresses from which the ticket can be
2314
used. If there are no addresses, the ticket can be used from any
2315
location. The decision by the KDC to issue or by the end server to
2316
accept zero-address tickets is a policy decision and is left to the
2317
Kerberos and end-service administrators; they may refuse to issue or
2318
accept such tickets. The suggested and default policy, however, is that
2319
such tickets will only be issued or accepted when additional
2320
information that can be used to restrict the use of the ticket is
2321
included in the authorization_data field. Such a ticket is a
2322
capability.
2323
2324
Network addresses are included in the ticket to make it harder for an
2325
attacker to use stolen credentials. Because the session key is not sent
2326
over the network in cleartext, credentials can't be stolen simply by
2327
listening to the network; an attacker has to gain access to the session
2328
key (perhaps through operating system security breaches or a careless
2329
user's unattended session) to make use of stolen tickets.
2330
2331
It is important to note that the network address from which a
2332
connection is received cannot be reliably determined. Even if it could
2333
be, an attacker who has compromised the client's workstation could use
2334
the credentials from there. Including the network addresses only makes
2335
it more difficult, not impossible, for an attacker to walk off with
2336
stolen credentials and then use them from a "safe" location.
2337
authorization-data
2338
The authorization-data field is used to pass authorization data from
2339
2340
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2341
2342
2343
2344
2345
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2346
2347
the principal on whose behalf a ticket was issued to the application
2348
service. If no authorization data is included, this field will be left
2349
out. Experience has shown that the name of this field is confusing, and
2350
that a better name for this field would be restrictions. Unfortunately,
2351
it is not possible to change the name of this field at this time.
2352
2353
This field contains restrictions on any authority obtained on the basis
2354
of authentication using the ticket. It is possible for any principal in
2355
posession of credentials to add entries to the authorization data field
2356
since these entries further restrict what can be done with the ticket.
2357
Such additions can be made by specifying the additional entries when a
2358
new ticket is obtained during the TGS exchange, or they may be added
2359
during chained delegation using the authorization data field of the
2360
authenticator.
2361
2362
Because entries may be added to this field by the holder of
2363
credentials, except when an entry is separately authenticated by
2364
encapulation in the kdc-issued element, it is not allowable for the
2365
presence of an entry in the authorization data field of a ticket to
2366
amplify the priveleges one would obtain from using a ticket.
2367
2368
The data in this field may be specific to the end service; the field
2369
will contain the names of service specific objects, and the rights to
2370
those objects. The format for this field is described in section 5.2.
2371
Although Kerberos is not concerned with the format of the contents of
2372
the sub-fields, it does carry type information (ad-type).
2373
2374
By using the authorization_data field, a principal is able to issue a
2375
proxy that is valid for a specific purpose. For example, a client
2376
wishing to print a file can obtain a file server proxy to be passed to
2377
the print server. By specifying the name of the file in the
2378
authorization_data field, the file server knows that the print server
2379
can only use the client's rights when accessing the particular file to
2380
be printed.
2381
2382
A separate service providing authorization or certifying group
2383
membership may be built using the authorization-data field. In this
2384
case, the entity granting authorization (not the authorized entity),
2385
may obtain a ticket in its own name (e.g. the ticket is issued in the
2386
name of a privelege server), and this entity adds restrictions on its
2387
own authority and delegates the restricted authority through a proxy to
2388
the client. The client would then present this authorization credential
2389
to the application server separately from the authentication exchange.
2390
Alternatively, such authorization credentials may be embedded in the
2391
ticket authenticating the authorized entity, when the authorization is
2392
separately authenticated using the kdc-issued authorization data
2393
element (see B.4).
2394
2395
Similarly, if one specifies the authorization-data field of a proxy and
2396
leaves the host addresses blank, the resulting ticket and session key
2397
can be treated as a capability. See [Neu93] for some suggested uses of
2398
this field.
2399
2400
The authorization-data field is optional and does not have to be
2401
included in a ticket.
2402
2403
5.3.2. Authenticators
2404
2405
An authenticator is a record sent with a ticket to a server to certify the
2406
2407
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2408
2409
2410
2411
2412
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2413
2414
client's knowledge of the encryption key in the ticket, to help the server
2415
detect replays, and to help choose a "true session key" to use with the
2416
particular session. The encoding is encrypted in the ticket's session key
2417
shared by the client and the server:
2418
2419
-- Unencrypted authenticator
2420
Authenticator ::= [APPLICATION 2] SEQUENCE {
2421
authenticator-vno[0] INTEGER,
2422
crealm[1] Realm,
2423
cname[2] PrincipalName,
2424
cksum[3] Checksum OPTIONAL,
2425
cusec[4] INTEGER,
2426
ctime[5] KerberosTime,
2427
subkey[6] EncryptionKey OPTIONAL,
2428
seq-number[7] INTEGER OPTIONAL,
2429
authorization-data[8] AuthorizationData OPTIONAL
2430
}
2431
2432
2433
authenticator-vno
2434
This field specifies the version number for the format of the
2435
authenticator. This document specifies version 5.
2436
crealm and cname
2437
These fields are the same as those described for the ticket in section
2438
5.3.1.
2439
cksum
2440
This field contains a checksum of the the applica- tion data that
2441
accompanies the KRB_AP_REQ.
2442
cusec
2443
This field contains the microsecond part of the client's timestamp. Its
2444
value (before encryption) ranges from 0 to 999999. It often appears
2445
along with ctime. The two fields are used together to specify a
2446
reasonably accurate timestamp.
2447
ctime
2448
This field contains the current time on the client's host.
2449
subkey
2450
This field contains the client's choice for an encryption key which is
2451
to be used to protect this specific application session. Unless an
2452
application specifies otherwise, if this field is left out the session
2453
key from the ticket will be used.
2454
seq-number
2455
This optional field includes the initial sequence number to be used by
2456
the KRB_PRIV or KRB_SAFE messages when sequence numbers are used to
2457
detect replays (It may also be used by application specific messages).
2458
When included in the authenticator this field specifies the initial
2459
sequence number for messages from the client to the server. When
2460
included in the AP-REP message, the initial sequence number is that for
2461
messages from the server to the client. When used in KRB_PRIV or
2462
KRB_SAFE messages, it is incremented by one after each message is sent.
2463
Sequence numbers fall in the range of 0 through 2^32 - 1 and wrap to
2464
zero following the value 2^32 - 1.
2465
2466
For sequence numbers to adequately support the detection of replays
2467
they should be non-repeating, even across connection boundaries. The
2468
initial sequence number should be random and uniformly distributed
2469
across the full space of possible sequence numbers, so that it cannot
2470
be guessed by an attacker and so that it and the successive sequence
2471
numbers do not repeat other sequences.
2472
authorization-data
2473
2474
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2475
2476
2477
2478
2479
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2480
2481
This field is the same as described for the ticket in section 5.3.1. It
2482
is optional and will only appear when additional restrictions are to be
2483
placed on the use of a ticket, beyond those carried in the ticket
2484
itself.
2485
2486
5.4. Specifications for the AS and TGS exchanges
2487
2488
This section specifies the format of the messages used in the exchange
2489
between the client and the Kerberos server. The format of possible error
2490
messages appears in section 5.9.1.
2491
2492
5.4.1. KRB_KDC_REQ definition
2493
2494
The KRB_KDC_REQ message has no type of its own. Instead, its type is one of
2495
KRB_AS_REQ or KRB_TGS_REQ depending on whether the request is for an initial
2496
ticket or an additional ticket. In either case, the message is sent from the
2497
client to the Authentication Server to request credentials for a service.
2498
2499
The message fields are:
2500
2501
AS-REQ ::= [APPLICATION 10] KDC-REQ
2502
TGS-REQ ::= [APPLICATION 12] KDC-REQ
2503
2504
KDC-REQ ::= SEQUENCE {
2505
pvno[1] INTEGER,
2506
msg-type[2] INTEGER,
2507
padata[3] SEQUENCE OF PA-DATA OPTIONAL,
2508
req-body[4] KDC-REQ-BODY
2509
}
2510
2511
PA-DATA ::= SEQUENCE {
2512
padata-type[1] INTEGER,
2513
padata-value[2] OCTET STRING,
2514
-- might be encoded AP-REQ
2515
}
2516
2517
KDC-REQ-BODY ::= SEQUENCE {
2518
kdc-options[0] KDCOptions,
2519
cname[1] PrincipalName OPTIONAL,
2520
-- Used only in AS-REQ
2521
realm[2] Realm, -- Server's realm
2522
-- Also client's in AS-REQ
2523
sname[3] PrincipalName OPTIONAL,
2524
from[4] KerberosTime OPTIONAL,
2525
till[5] KerberosTime OPTIONAL,
2526
rtime[6] KerberosTime OPTIONAL,
2527
nonce[7] INTEGER,
2528
etype[8] SEQUENCE OF INTEGER,
2529
-- EncryptionType,
2530
-- in preference order
2531
addresses[9] HostAddresses OPTIONAL,
2532
enc-authorization-data[10] EncryptedData OPTIONAL,
2533
-- Encrypted AuthorizationData
2534
-- encoding
2535
additional-tickets[11] SEQUENCE OF Ticket OPTIONAL
2536
}
2537
2538
The fields in this message are:
2539
2540
2541
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2542
2543
2544
2545
2546
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2547
2548
pvno
2549
This field is included in each message, and specifies the protocol
2550
version number. This document specifies protocol version 5.
2551
msg-type
2552
This field indicates the type of a protocol message. It will almost
2553
always be the same as the application identifier associated with a
2554
message. It is included to make the identifier more readily accessible
2555
to the application. For the KDC-REQ message, this type will be
2556
KRB_AS_REQ or KRB_TGS_REQ.
2557
padata
2558
The padata (pre-authentication data) field contains a sequence of
2559
authentication information which may be needed before credentials can
2560
be issued or decrypted. In the case of requests for additional tickets
2561
(KRB_TGS_REQ), this field will include an element with padata-type of
2562
PA-TGS-REQ and data of an authentication header (ticket-granting ticket
2563
and authenticator). The checksum in the authenticator (which must be
2564
collision-proof) is to be computed over the KDC-REQ-BODY encoding. In
2565
most requests for initial authentication (KRB_AS_REQ) and most replies
2566
(KDC-REP), the padata field will be left out.
2567
2568
This field may also contain information needed by certain extensions to
2569
the Kerberos protocol. For example, it might be used to initially
2570
verify the identity of a client before any response is returned. This
2571
is accomplished with a padata field with padata-type equal to
2572
PA-ENC-TIMESTAMP and padata-value defined as follows:
2573
2574
padata-type ::= PA-ENC-TIMESTAMP
2575
padata-value ::= EncryptedData -- PA-ENC-TS-ENC
2576
2577
PA-ENC-TS-ENC ::= SEQUENCE {
2578
patimestamp[0] KerberosTime, -- client's time
2579
pausec[1] INTEGER OPTIONAL
2580
}
2581
2582
with patimestamp containing the client's time and pausec containing the
2583
microseconds which may be omitted if a client will not generate more
2584
than one request per second. The ciphertext (padata-value) consists of
2585
the PA-ENC-TS-ENC sequence, encrypted using the client's secret key.
2586
2587
[use-specified-kvno item is here for discussion and may be removed] It
2588
may also be used by the client to specify the version of a key that is
2589
being used for accompanying preauthentication, and/or which should be
2590
used to encrypt the reply from the KDC.
2591
2592
PA-USE-SPECIFIED-KVNO ::= Integer
2593
2594
The KDC should only accept and abide by the value of the
2595
use-specified-kvno preauthentication data field when the specified key
2596
is still valid and until use of a new key is confirmed. This situation
2597
is likely to occur primarily during the period during which an updated
2598
key is propagating to other KDC's in a realm.
2599
2600
The padata field can also contain information needed to help the KDC or
2601
the client select the key needed for generating or decrypting the
2602
response. This form of the padata is useful for supporting the use of
2603
certain token cards with Kerberos. The details of such extensions are
2604
specified in separate documents. See [Pat92] for additional uses of
2605
this field.
2606
padata-type
2607
2608
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2609
2610
2611
2612
2613
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2614
2615
The padata-type element of the padata field indicates the way that the
2616
padata-value element is to be interpreted. Negative values of
2617
padata-type are reserved for unregistered use; non-negative values are
2618
used for a registered interpretation of the element type.
2619
req-body
2620
This field is a placeholder delimiting the extent of the remaining
2621
fields. If a checksum is to be calculated over the request, it is
2622
calculated over an encoding of the KDC-REQ-BODY sequence which is
2623
enclosed within the req-body field.
2624
kdc-options
2625
This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to the
2626
KDC and indicates the flags that the client wants set on the tickets as
2627
well as other information that is to modify the behavior of the KDC.
2628
Where appropriate, the name of an option may be the same as the flag
2629
that is set by that option. Although in most case, the bit in the
2630
options field will be the same as that in the flags field, this is not
2631
guaranteed, so it is not acceptable to simply copy the options field to
2632
the flags field. There are various checks that must be made before
2633
honoring an option anyway.
2634
2635
The kdc_options field is a bit-field, where the selected options are
2636
indicated by the bit being set (1), and the unselected options and
2637
reserved fields being reset (0). The encoding of the bits is specified
2638
in section 5.2. The options are described in more detail above in
2639
section 2. The meanings of the options are:
2640
2641
Bit(s) Name Description
2642
0 RESERVED
2643
Reserved for future expansion of this
2644
field.
2645
2646
1 FORWARDABLE
2647
The FORWARDABLE option indicates that
2648
the ticket to be issued is to have its
2649
forwardable flag set. It may only be
2650
set on the initial request, or in a sub-
2651
sequent request if the ticket-granting
2652
ticket on which it is based is also for-
2653
wardable.
2654
2655
2 FORWARDED
2656
The FORWARDED option is only specified
2657
in a request to the ticket-granting
2658
server and will only be honored if the
2659
ticket-granting ticket in the request
2660
has its FORWARDABLE bit set. This
2661
option indicates that this is a request
2662
for forwarding. The address(es) of the
2663
host from which the resulting ticket is
2664
to be valid are included in the
2665
addresses field of the request.
2666
2667
3 PROXIABLE
2668
The PROXIABLE option indicates that the
2669
ticket to be issued is to have its prox-
2670
iable flag set. It may only be set on
2671
the initial request, or in a subsequent
2672
request if the ticket-granting ticket on
2673
which it is based is also proxiable.
2674
2675
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2676
2677
2678
2679
2680
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2681
2682
2683
4 PROXY
2684
The PROXY option indicates that this is
2685
a request for a proxy. This option will
2686
only be honored if the ticket-granting
2687
ticket in the request has its PROXIABLE
2688
bit set. The address(es) of the host
2689
from which the resulting ticket is to be
2690
valid are included in the addresses
2691
field of the request.
2692
2693
5 ALLOW-POSTDATE
2694
The ALLOW-POSTDATE option indicates that
2695
the ticket to be issued is to have its
2696
MAY-POSTDATE flag set. It may only be
2697
set on the initial request, or in a sub-
2698
sequent request if the ticket-granting
2699
ticket on which it is based also has its
2700
MAY-POSTDATE flag set.
2701
2702
6 POSTDATED
2703
The POSTDATED option indicates that this
2704
is a request for a postdated ticket.
2705
This option will only be honored if the
2706
ticket-granting ticket on which it is
2707
based has its MAY-POSTDATE flag set.
2708
The resulting ticket will also have its
2709
INVALID flag set, and that flag may be
2710
reset by a subsequent request to the KDC
2711
after the starttime in the ticket has
2712
been reached.
2713
2714
7 UNUSED
2715
This option is presently unused.
2716
2717
8 RENEWABLE
2718
The RENEWABLE option indicates that the
2719
ticket to be issued is to have its
2720
RENEWABLE flag set. It may only be set
2721
on the initial request, or when the
2722
ticket-granting ticket on which the
2723
request is based is also renewable. If
2724
this option is requested, then the rtime
2725
field in the request contains the
2726
desired absolute expiration time for the
2727
ticket.
2728
2729
9-13 UNUSED
2730
These options are presently unused.
2731
2732
14 REQUEST-ANONYMOUS
2733
The REQUEST-ANONYMOUS option indicates
2734
that the ticket to be issued is not to
2735
identify the user to which it was
2736
issued. Instead, the principal identif-
2737
ier is to be generic, as specified by
2738
the policy of the realm (e.g. usually
2739
anonymous@realm). The purpose of the
2740
ticket is only to securely distribute a
2741
2742
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2743
2744
2745
2746
2747
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2748
2749
session key, and not to identify the
2750
user. The ANONYMOUS flag on the ticket
2751
to be returned should be set. If the
2752
local realms policy does not permit
2753
anonymous credentials, the request is to
2754
be rejected.
2755
2756
15-25 RESERVED
2757
Reserved for future use.
2758
2759
26 DISABLE-TRANSITED-CHECK
2760
By default the KDC will check the
2761
transited field of a ticket-granting-
2762
ticket against the policy of the local
2763
realm before it will issue derivative
2764
tickets based on the ticket granting
2765
ticket. If this flag is set in the
2766
request, checking of the transited field
2767
is disabled. Tickets issued without the
2768
performance of this check will be noted
2769
by the reset (0) value of the
2770
TRANSITED-POLICY-CHECKED flag,
2771
indicating to the application server
2772
that the tranisted field must be checked
2773
locally. KDC's are encouraged but not
2774
required to honor the
2775
DISABLE-TRANSITED-CHECK option.
2776
2777
27 RENEWABLE-OK
2778
The RENEWABLE-OK option indicates that a
2779
renewable ticket will be acceptable if a
2780
ticket with the requested life cannot
2781
otherwise be provided. If a ticket with
2782
the requested life cannot be provided,
2783
then a renewable ticket may be issued
2784
with a renew-till equal to the the
2785
requested endtime. The value of the
2786
renew-till field may still be limited by
2787
local limits, or limits selected by the
2788
individual principal or server.
2789
2790
28 ENC-TKT-IN-SKEY
2791
This option is used only by the ticket-
2792
granting service. The ENC-TKT-IN-SKEY
2793
option indicates that the ticket for the
2794
end server is to be encrypted in the
2795
session key from the additional ticket-
2796
granting ticket provided.
2797
2798
29 RESERVED
2799
Reserved for future use.
2800
2801
30 RENEW
2802
This option is used only by the ticket-
2803
granting service. The RENEW option
2804
indicates that the present request is
2805
for a renewal. The ticket provided is
2806
encrypted in the secret key for the
2807
server on which it is valid. This
2808
2809
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2810
2811
2812
2813
2814
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2815
2816
option will only be honored if the
2817
ticket to be renewed has its RENEWABLE
2818
flag set and if the time in its renew-
2819
till field has not passed. The ticket
2820
to be renewed is passed in the padata
2821
field as part of the authentication
2822
header.
2823
2824
31 VALIDATE
2825
This option is used only by the ticket-
2826
granting service. The VALIDATE option
2827
indicates that the request is to vali-
2828
date a postdated ticket. It will only
2829
be honored if the ticket presented is
2830
postdated, presently has its INVALID
2831
flag set, and would be otherwise usable
2832
at this time. A ticket cannot be vali-
2833
dated before its starttime. The ticket
2834
presented for validation is encrypted in
2835
the key of the server for which it is
2836
valid and is passed in the padata field
2837
as part of the authentication header.
2838
2839
cname and sname
2840
These fields are the same as those described for the ticket in section
2841
5.3.1. sname may only be absent when the ENC-TKT-IN-SKEY option is
2842
specified. If absent, the name of the server is taken from the name of
2843
the client in the ticket passed as additional-tickets.
2844
enc-authorization-data
2845
The enc-authorization-data, if present (and it can only be present in
2846
the TGS_REQ form), is an encoding of the desired authorization-data
2847
encrypted under the sub-session key if present in the Authenticator, or
2848
alternatively from the session key in the ticket-granting ticket, both
2849
from the padata field in the KRB_AP_REQ.
2850
realm
2851
This field specifies the realm part of the server's principal
2852
identifier. In the AS exchange, this is also the realm part of the
2853
client's principal identifier.
2854
from
2855
This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
2856
requests when the requested ticket is to be postdated. It specifies the
2857
desired start time for the requested ticket. If this field is omitted
2858
then the KDC should use the current time instead.
2859
till
2860
This field contains the expiration date requested by the client in a
2861
ticket request. It is optional and if omitted the requested ticket is
2862
to have the maximum endtime permitted according to KDC policy for the
2863
parties to the authentication exchange as limited by expiration date of
2864
the ticket granting ticket or other preauthentication credentials.
2865
rtime
2866
This field is the requested renew-till time sent from a client to the
2867
KDC in a ticket request. It is optional.
2868
nonce
2869
This field is part of the KDC request and response. It it intended to
2870
hold a random number generated by the client. If the same number is
2871
included in the encrypted response from the KDC, it provides evidence
2872
that the response is fresh and has not been replayed by an attacker.
2873
Nonces must never be re-used. Ideally, it should be generated randomly,
2874
but if the correct time is known, it may suffice[25].
2875
2876
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2877
2878
2879
2880
2881
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2882
2883
etype
2884
This field specifies the desired encryption algorithm to be used in the
2885
response.
2886
addresses
2887
This field is included in the initial request for tickets, and
2888
optionally included in requests for additional tickets from the
2889
ticket-granting server. It specifies the addresses from which the
2890
requested ticket is to be valid. Normally it includes the addresses for
2891
the client's host. If a proxy is requested, this field will contain
2892
other addresses. The contents of this field are usually copied by the
2893
KDC into the caddr field of the resulting ticket.
2894
additional-tickets
2895
Additional tickets may be optionally included in a request to the
2896
ticket-granting server. If the ENC-TKT-IN-SKEY option has been
2897
specified, then the session key from the additional ticket will be used
2898
in place of the server's key to encrypt the new ticket. If more than
2899
one option which requires additional tickets has been specified, then
2900
the additional tickets are used in the order specified by the ordering
2901
of the options bits (see kdc-options, above).
2902
2903
The application code will be either ten (10) or twelve (12) depending on
2904
whether the request is for an initial ticket (AS-REQ) or for an additional
2905
ticket (TGS-REQ).
2906
2907
The optional fields (addresses, authorization-data and additional-tickets)
2908
are only included if necessary to perform the operation specified in the
2909
kdc-options field.
2910
2911
It should be noted that in KRB_TGS_REQ, the protocol version number appears
2912
twice and two different message types appear: the KRB_TGS_REQ message
2913
contains these fields as does the authentication header (KRB_AP_REQ) that is
2914
passed in the padata field.
2915
2916
5.4.2. KRB_KDC_REP definition
2917
2918
The KRB_KDC_REP message format is used for the reply from the KDC for either
2919
an initial (AS) request or a subsequent (TGS) request. There is no message
2920
type for KRB_KDC_REP. Instead, the type will be either KRB_AS_REP or
2921
KRB_TGS_REP. The key used to encrypt the ciphertext part of the reply
2922
depends on the message type. For KRB_AS_REP, the ciphertext is encrypted in
2923
the client's secret key, and the client's key version number is included in
2924
the key version number for the encrypted data. For KRB_TGS_REP, the
2925
ciphertext is encrypted in the sub-session key from the Authenticator, or if
2926
absent, the session key from the ticket-granting ticket used in the request.
2927
In that case, no version number will be present in the EncryptedData
2928
sequence.
2929
2930
The KRB_KDC_REP message contains the following fields:
2931
2932
AS-REP ::= [APPLICATION 11] KDC-REP
2933
TGS-REP ::= [APPLICATION 13] KDC-REP
2934
2935
KDC-REP ::= SEQUENCE {
2936
pvno[0] INTEGER,
2937
msg-type[1] INTEGER,
2938
padata[2] SEQUENCE OF PA-DATA OPTIONAL,
2939
crealm[3] Realm,
2940
cname[4] PrincipalName,
2941
ticket[5] Ticket,
2942
2943
Neuman, Ts'o, Kohl Expires: 10 September, 2000
2944
2945
2946
2947
2948
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
2949
2950
enc-part[6] EncryptedData
2951
}
2952
2953
EncASRepPart ::= [APPLICATION 25[27]] EncKDCRepPart
2954
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
2955
2956
EncKDCRepPart ::= SEQUENCE {
2957
key[0] EncryptionKey,
2958
last-req[1] LastReq,
2959
nonce[2] INTEGER,
2960
key-expiration[3] KerberosTime OPTIONAL,
2961
flags[4] TicketFlags,
2962
authtime[5] KerberosTime,
2963
starttime[6] KerberosTime OPTIONAL,
2964
endtime[7] KerberosTime,
2965
renew-till[8] KerberosTime OPTIONAL,
2966
srealm[9] Realm,
2967
sname[10] PrincipalName,
2968
caddr[11] HostAddresses OPTIONAL
2969
}
2970
2971
pvno and msg-type
2972
These fields are described above in section 5.4.1. msg-type is either
2973
KRB_AS_REP or KRB_TGS_REP.
2974
padata
2975
This field is described in detail in section 5.4.1. One possible use
2976
for this field is to encode an alternate "mix-in" string to be used
2977
with a string-to-key algorithm (such as is described in section 6.3.2).
2978
This ability is useful to ease transitions if a realm name needs to
2979
change (e.g. when a company is acquired); in such a case all existing
2980
password-derived entries in the KDC database would be flagged as
2981
needing a special mix-in string until the next password change.
2982
crealm, cname, srealm and sname
2983
These fields are the same as those described for the ticket in section
2984
5.3.1.
2985
ticket
2986
The newly-issued ticket, from section 5.3.1.
2987
enc-part
2988
This field is a place holder for the ciphertext and related information
2989
that forms the encrypted part of a message. The description of the
2990
encrypted part of the message follows each appearance of this field.
2991
The encrypted part is encoded as described in section 6.1.
2992
key
2993
This field is the same as described for the ticket in section 5.3.1.
2994
last-req
2995
This field is returned by the KDC and specifies the time(s) of the last
2996
request by a principal. Depending on what information is available,
2997
this might be the last time that a request for a ticket-granting ticket
2998
was made, or the last time that a request based on a ticket-granting
2999
ticket was successful. It also might cover all servers for a realm, or
3000
just the particular server. Some implementations may display this
3001
information to the user to aid in discovering unauthorized use of one's
3002
identity. It is similar in spirit to the last login time displayed when
3003
logging into timesharing systems.
3004
nonce
3005
This field is described above in section 5.4.1.
3006
key-expiration
3007
The key-expiration field is part of the response from the KDC and
3008
specifies the time that the client's secret key is due to expire. The
3009
3010
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3011
3012
3013
3014
3015
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3016
3017
expiration might be the result of password aging or an account
3018
expiration. This field will usually be left out of the TGS reply since
3019
the response to the TGS request is encrypted in a session key and no
3020
client information need be retrieved from the KDC database. It is up to
3021
the application client (usually the login program) to take appropriate
3022
action (such as notifying the user) if the expiration time is imminent.
3023
flags, authtime, starttime, endtime, renew-till and caddr
3024
These fields are duplicates of those found in the encrypted portion of
3025
the attached ticket (see section 5.3.1), provided so the client may
3026
verify they match the intended request and to assist in proper ticket
3027
caching. If the message is of type KRB_TGS_REP, the caddr field will
3028
only be filled in if the request was for a proxy or forwarded ticket,
3029
or if the user is substituting a subset of the addresses from the
3030
ticket granting ticket. If the client-requested addresses are not
3031
present or not used, then the addresses contained in the ticket will be
3032
the same as those included in the ticket-granting ticket.
3033
3034
5.5. Client/Server (CS) message specifications
3035
3036
This section specifies the format of the messages used for the
3037
authentication of the client to the application server.
3038
3039
5.5.1. KRB_AP_REQ definition
3040
3041
The KRB_AP_REQ message contains the Kerberos protocol version number, the
3042
message type KRB_AP_REQ, an options field to indicate any options in use,
3043
and the ticket and authenticator themselves. The KRB_AP_REQ message is often
3044
referred to as the 'authentication header'.
3045
3046
AP-REQ ::= [APPLICATION 14] SEQUENCE {
3047
pvno[0] INTEGER,
3048
msg-type[1] INTEGER,
3049
ap-options[2] APOptions,
3050
ticket[3] Ticket,
3051
authenticator[4] EncryptedData
3052
}
3053
3054
APOptions ::= BIT STRING {
3055
reserved(0),
3056
use-session-key(1),
3057
mutual-required(2)
3058
}
3059
3060
3061
3062
pvno and msg-type
3063
These fields are described above in section 5.4.1. msg-type is
3064
KRB_AP_REQ.
3065
ap-options
3066
This field appears in the application request (KRB_AP_REQ) and affects
3067
the way the request is processed. It is a bit-field, where the selected
3068
options are indicated by the bit being set (1), and the unselected
3069
options and reserved fields being reset (0). The encoding of the bits
3070
is specified in section 5.2. The meanings of the options are:
3071
3072
Bit(s) Name Description
3073
3074
0 RESERVED
3075
Reserved for future expansion of this
3076
3077
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3078
3079
3080
3081
3082
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3083
3084
field.
3085
3086
1 USE-SESSION-KEY
3087
The USE-SESSION-KEY option indicates
3088
that the ticket the client is presenting
3089
to a server is encrypted in the session
3090
key from the server's ticket-granting
3091
ticket. When this option is not speci-
3092
fied, the ticket is encrypted in the
3093
server's secret key.
3094
3095
2 MUTUAL-REQUIRED
3096
The MUTUAL-REQUIRED option tells the
3097
server that the client requires mutual
3098
authentication, and that it must respond
3099
with a KRB_AP_REP message.
3100
3101
3-31 RESERVED
3102
Reserved for future use.
3103
3104
ticket
3105
This field is a ticket authenticating the client to the server.
3106
authenticator
3107
This contains the authenticator, which includes the client's choice of
3108
a subkey. Its encoding is described in section 5.3.2.
3109
3110
5.5.2. KRB_AP_REP definition
3111
3112
The KRB_AP_REP message contains the Kerberos protocol version number, the
3113
message type, and an encrypted time- stamp. The message is sent in in
3114
response to an application request (KRB_AP_REQ) where the mutual
3115
authentication option has been selected in the ap-options field.
3116
3117
AP-REP ::= [APPLICATION 15] SEQUENCE {
3118
pvno[0] INTEGER,
3119
msg-type[1] INTEGER,
3120
enc-part[2] EncryptedData
3121
}
3122
3123
EncAPRepPart ::= [APPLICATION 27[29]] SEQUENCE {
3124
ctime[0] KerberosTime,
3125
cusec[1] INTEGER,
3126
subkey[2] EncryptionKey OPTIONAL,
3127
seq-number[3] INTEGER OPTIONAL
3128
}
3129
3130
The encoded EncAPRepPart is encrypted in the shared session key of the
3131
ticket. The optional subkey field can be used in an application-arranged
3132
negotiation to choose a per association session key.
3133
3134
pvno and msg-type
3135
These fields are described above in section 5.4.1. msg-type is
3136
KRB_AP_REP.
3137
enc-part
3138
This field is described above in section 5.4.2.
3139
ctime
3140
This field contains the current time on the client's host.
3141
cusec
3142
This field contains the microsecond part of the client's timestamp.
3143
3144
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3145
3146
3147
3148
3149
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3150
3151
subkey
3152
This field contains an encryption key which is to be used to protect
3153
this specific application session. See section 3.2.6 for specifics on
3154
how this field is used to negotiate a key. Unless an application
3155
specifies otherwise, if this field is left out, the sub-session key
3156
from the authenticator, or if also left out, the session key from the
3157
ticket will be used.
3158
3159
5.5.3. Error message reply
3160
3161
If an error occurs while processing the application request, the KRB_ERROR
3162
message will be sent in response. See section 5.9.1 for the format of the
3163
error message. The cname and crealm fields may be left out if the server
3164
cannot determine their appropriate values from the corresponding KRB_AP_REQ
3165
message. If the authenticator was decipherable, the ctime and cusec fields
3166
will contain the values from it.
3167
3168
5.6. KRB_SAFE message specification
3169
3170
This section specifies the format of a message that can be used by either
3171
side (client or server) of an application to send a tamper-proof message to
3172
its peer. It presumes that a session key has previously been exchanged (for
3173
example, by using the KRB_AP_REQ/KRB_AP_REP messages).
3174
3175
5.6.1. KRB_SAFE definition
3176
3177
The KRB_SAFE message contains user data along with a collision-proof
3178
checksum keyed with the last encryption key negotiated via subkeys, or the
3179
session key if no negotiation has occured. The message fields are:
3180
3181
KRB-SAFE ::= [APPLICATION 20] SEQUENCE {
3182
pvno[0] INTEGER,
3183
msg-type[1] INTEGER,
3184
safe-body[2] KRB-SAFE-BODY,
3185
cksum[3] Checksum
3186
}
3187
3188
KRB-SAFE-BODY ::= SEQUENCE {
3189
user-data[0] OCTET STRING,
3190
timestamp[1] KerberosTime OPTIONAL,
3191
usec[2] INTEGER OPTIONAL,
3192
seq-number[3] INTEGER OPTIONAL,
3193
s-address[4] HostAddress OPTIONAL,
3194
r-address[5] HostAddress OPTIONAL
3195
}
3196
3197
pvno and msg-type
3198
These fields are described above in section 5.4.1. msg-type is
3199
KRB_SAFE.
3200
safe-body
3201
This field is a placeholder for the body of the KRB-SAFE message.
3202
cksum
3203
This field contains the checksum of the application data. Checksum
3204
details are described in section 6.4. The checksum is computed over the
3205
encoding of the KRB-SAFE sequence. First, the cksum is zeroed and the
3206
checksum is computed over the encoding of the KRB-SAFE sequence, then
3207
the checksum is set to the result of that computation, and finally the
3208
KRB-SAFE sequence is encoded again.
3209
user-data
3210
3211
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3212
3213
3214
3215
3216
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3217
3218
This field is part of the KRB_SAFE and KRB_PRIV messages and contain
3219
the application specific data that is being passed from the sender to
3220
the recipient.
3221
timestamp
3222
This field is part of the KRB_SAFE and KRB_PRIV messages. Its contents
3223
are the current time as known by the sender of the message. By checking
3224
the timestamp, the recipient of the message is able to make sure that
3225
it was recently generated, and is not a replay.
3226
usec
3227
This field is part of the KRB_SAFE and KRB_PRIV headers. It contains
3228
the microsecond part of the timestamp.
3229
seq-number
3230
This field is described above in section 5.3.2.
3231
s-address
3232
This field specifies the address in use by the sender of the message.
3233
It may be omitted if not required by the application protocol. The
3234
application designer considering omission of this field is warned, that
3235
the inclusion of this address prevents some kinds of replay attacks
3236
(e.g., reflection attacks) and that it is only acceptable to omit this
3237
address if there is sufficient information in the integrity protected
3238
part of the application message for the recipient to unambiguously
3239
determine if it was the intended recipient.
3240
r-address
3241
This field specifies the address in use by the recipient of the
3242
message. It may be omitted for some uses (such as broadcast protocols),
3243
but the recipient may arbitrarily reject such messages. This field
3244
along with s-address can be used to help detect messages which have
3245
been incorrectly or maliciously delivered to the wrong recipient.
3246
3247
5.7. KRB_PRIV message specification
3248
3249
This section specifies the format of a message that can be used by either
3250
side (client or server) of an application to securely and privately send a
3251
message to its peer. It presumes that a session key has previously been
3252
exchanged (for example, by using the KRB_AP_REQ/KRB_AP_REP messages).
3253
3254
5.7.1. KRB_PRIV definition
3255
3256
The KRB_PRIV message contains user data encrypted in the Session Key. The
3257
message fields are:
3258
3259
KRB-PRIV ::= [APPLICATION 21] SEQUENCE {
3260
pvno[0] INTEGER,
3261
msg-type[1] INTEGER,
3262
enc-part[3] EncryptedData
3263
}
3264
3265
EncKrbPrivPart ::= [APPLICATION 28[31]] SEQUENCE {
3266
user-data[0] OCTET STRING,
3267
timestamp[1] KerberosTime OPTIONAL,
3268
usec[2] INTEGER OPTIONAL,
3269
seq-number[3] INTEGER OPTIONAL,
3270
s-address[4] HostAddress OPTIONAL, -- sender's addr
3271
r-address[5] HostAddress OPTIONAL -- recip's addr
3272
}
3273
3274
pvno and msg-type
3275
These fields are described above in section 5.4.1. msg-type is
3276
KRB_PRIV.
3277
3278
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3279
3280
3281
3282
3283
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3284
3285
enc-part
3286
This field holds an encoding of the EncKrbPrivPart sequence encrypted
3287
under the session key[32]. This encrypted encoding is used for the
3288
enc-part field of the KRB-PRIV message. See section 6 for the format of
3289
the ciphertext.
3290
user-data, timestamp, usec, s-address and r-address
3291
These fields are described above in section 5.6.1.
3292
seq-number
3293
This field is described above in section 5.3.2.
3294
3295
5.8. KRB_CRED message specification
3296
3297
This section specifies the format of a message that can be used to send
3298
Kerberos credentials from one principal to another. It is presented here to
3299
encourage a common mechanism to be used by applications when forwarding
3300
tickets or providing proxies to subordinate servers. It presumes that a
3301
session key has already been exchanged perhaps by using the
3302
KRB_AP_REQ/KRB_AP_REP messages.
3303
3304
5.8.1. KRB_CRED definition
3305
3306
The KRB_CRED message contains a sequence of tickets to be sent and
3307
information needed to use the tickets, including the session key from each.
3308
The information needed to use the tickets is encrypted under an encryption
3309
key previously exchanged or transferred alongside the KRB_CRED message. The
3310
message fields are:
3311
3312
KRB-CRED ::= [APPLICATION 22] SEQUENCE {
3313
pvno[0] INTEGER,
3314
msg-type[1] INTEGER, -- KRB_CRED
3315
tickets[2] SEQUENCE OF Ticket,
3316
enc-part[3] EncryptedData
3317
}
3318
3319
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
3320
ticket-info[0] SEQUENCE OF KrbCredInfo,
3321
nonce[1] INTEGER OPTIONAL,
3322
timestamp[2] KerberosTime OPTIONAL,
3323
usec[3] INTEGER OPTIONAL,
3324
s-address[4] HostAddress OPTIONAL,
3325
r-address[5] HostAddress OPTIONAL
3326
}
3327
3328
KrbCredInfo ::= SEQUENCE {
3329
key[0] EncryptionKey,
3330
prealm[1] Realm OPTIONAL,
3331
pname[2] PrincipalName OPTIONAL,
3332
flags[3] TicketFlags OPTIONAL,
3333
authtime[4] KerberosTime OPTIONAL,
3334
starttime[5] KerberosTime OPTIONAL,
3335
endtime[6] KerberosTime OPTIONAL
3336
renew-till[7] KerberosTime OPTIONAL,
3337
srealm[8] Realm OPTIONAL,
3338
sname[9] PrincipalName OPTIONAL,
3339
caddr[10] HostAddresses OPTIONAL
3340
}
3341
3342
pvno and msg-type
3343
These fields are described above in section 5.4.1. msg-type is
3344
3345
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3346
3347
3348
3349
3350
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3351
3352
KRB_CRED.
3353
tickets
3354
These are the tickets obtained from the KDC specifically for use by the
3355
intended recipient. Successive tickets are paired with the
3356
corresponding KrbCredInfo sequence from the enc-part of the KRB-CRED
3357
message.
3358
enc-part
3359
This field holds an encoding of the EncKrbCredPart sequence encrypted
3360
under the session key shared between the sender and the intended
3361
recipient. This encrypted encoding is used for the enc-part field of
3362
the KRB-CRED message. See section 6 for the format of the ciphertext.
3363
nonce
3364
If practical, an application may require the inclusion of a nonce
3365
generated by the recipient of the message. If the same value is
3366
included as the nonce in the message, it provides evidence that the
3367
message is fresh and has not been replayed by an attacker. A nonce must
3368
never be re-used; it should be generated randomly by the recipient of
3369
the message and provided to the sender of the message in an application
3370
specific manner.
3371
timestamp and usec
3372
These fields specify the time that the KRB-CRED message was generated.
3373
The time is used to provide assurance that the message is fresh.
3374
s-address and r-address
3375
These fields are described above in section 5.6.1. They are used
3376
optionally to provide additional assurance of the integrity of the
3377
KRB-CRED message.
3378
key
3379
This field exists in the corresponding ticket passed by the KRB-CRED
3380
message and is used to pass the session key from the sender to the
3381
intended recipient. The field's encoding is described in section 6.2.
3382
3383
The following fields are optional. If present, they can be associated with
3384
the credentials in the remote ticket file. If left out, then it is assumed
3385
that the recipient of the credentials already knows their value.
3386
3387
prealm and pname
3388
The name and realm of the delegated principal identity.
3389
flags, authtime, starttime, endtime, renew-till, srealm, sname, and caddr
3390
These fields contain the values of the correspond- ing fields from the
3391
ticket found in the ticket field. Descriptions of the fields are
3392
identical to the descriptions in the KDC-REP message.
3393
3394
5.9. Error message specification
3395
3396
This section specifies the format for the KRB_ERROR message. The fields
3397
included in the message are intended to return as much information as
3398
possible about an error. It is not expected that all the information
3399
required by the fields will be available for all types of errors. If the
3400
appropriate information is not available when the message is composed, the
3401
corresponding field will be left out of the message.
3402
3403
Note that since the KRB_ERROR message is only optionally integrity
3404
protected, it is quite possible for an intruder to synthesize or modify such
3405
a message. In particular, this means that unless appropriate integrity
3406
protection mechanisms have been applied to the KRB_ERROR message, the client
3407
should not use any fields in this message for security-critical purposes,
3408
such as setting a system clock or generating a fresh authenticator. The
3409
message can be useful, however, for advising a user on the reason for some
3410
failure.
3411
3412
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3413
3414
3415
3416
3417
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3418
3419
3420
5.9.1. KRB_ERROR definition
3421
3422
The KRB_ERROR message consists of the following fields:
3423
3424
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
3425
pvno[0] INTEGER,
3426
msg-type[1] INTEGER,
3427
ctime[2] KerberosTime OPTIONAL,
3428
cusec[3] INTEGER OPTIONAL,
3429
stime[4] KerberosTime,
3430
susec[5] INTEGER,
3431
error-code[6] INTEGER,
3432
crealm[7] Realm OPTIONAL,
3433
cname[8] PrincipalName OPTIONAL,
3434
realm[9] Realm, -- Correct realm
3435
sname[10] PrincipalName, -- Correct name
3436
e-text[11] GeneralString OPTIONAL,
3437
e-data[12] OCTET STRING OPTIONAL,
3438
e-cksum[13] Checksum OPTIONAL,
3439
}
3440
3441
3442
3443
pvno and msg-type
3444
These fields are described above in section 5.4.1. msg-type is
3445
KRB_ERROR.
3446
ctime
3447
This field is described above in section 5.4.1.
3448
cusec
3449
This field is described above in section 5.5.2.
3450
stime
3451
This field contains the current time on the server. It is of type
3452
KerberosTime.
3453
susec
3454
This field contains the microsecond part of the server's timestamp. Its
3455
value ranges from 0 to 999999. It appears along with stime. The two
3456
fields are used in conjunction to specify a reasonably accurate
3457
timestamp.
3458
error-code
3459
This field contains the error code returned by Kerberos or the server
3460
when a request fails. To interpret the value of this field see the list
3461
of error codes in section 8. Implementations are encouraged to provide
3462
for national language support in the display of error messages.
3463
crealm, cname, srealm and sname
3464
These fields are described above in section 5.3.1.
3465
e-text
3466
This field contains additional text to help explain the error code
3467
associated with the failed request (for example, it might include a
3468
principal name which was unknown).
3469
e-data
3470
This field contains additional data about the error for use by the
3471
application to help it recover from or handle the error. If present,
3472
this field will contain the encoding of a sequence of TypedData
3473
(TYPED-DATA below), unless the errorcode is KDC_ERR_PREAUTH_REQUIRED,
3474
in which case it will contain the encoding of a sequence of of padata
3475
fields (METHOD-DATA below), each corresponding to an acceptable
3476
pre-authentication method and optionally containing data for the
3477
method:
3478
3479
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3480
3481
3482
3483
3484
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3485
3486
3487
TYPED-DATA ::= SEQUENCE of TypeData
3488
METHOD-DATA ::= SEQUENCE of PA-DATA
3489
3490
TypedData ::= SEQUENCE {
3491
data-type[0] INTEGER,
3492
data-value[1] OCTET STRING OPTIONAL
3493
}
3494
3495
Note that e-data-types have been reserved for all PA data types defined
3496
prior to July 1999. For the KDC_ERR_PREAUTH_REQUIRED message, when
3497
using new PA data types defined in July 1999 or later, the METHOD-DATA
3498
sequence must itself be encapsulated in an TypedData element of type
3499
TD-PADATA. All new implementations interpreting the METHOD-DATA field
3500
for the KDC_ERR_PREAUTH_REQUIRED message must accept a type of
3501
TD-PADATA, extract the typed data field and interpret the use any
3502
elements encapsulated in the TD-PADATA elements as if they were present
3503
in the METHOD-DATA sequence.
3504
e-cksum
3505
This field contains an optional checksum for the KRB-ERROR message. The
3506
checksum is calculated over the Kerberos ASN.1 encoding of the
3507
KRB-ERROR message with the checksum absent. The checksum is then added
3508
to the KRB-ERROR structure and the message is re-encoded. The Checksum
3509
should be calculated using the session key from the ticket granting
3510
ticket or service ticket, where available. If the error is in response
3511
to a TGS or AP request, the checksum should be calculated uing the the
3512
session key from the client's ticket. If the error is in response to an
3513
AS request, then the checksum should be calulated using the client's
3514
secret key ONLY if there has been suitable preauthentication to prove
3515
knowledge of the secret key by the client[33]. If a checksum can not be
3516
computed because the key to be used is not available, no checksum will
3517
be included.
3518
3519
6. Encryption and Checksum Specifications
3520
3521
The Kerberos protocols described in this document are designed to use
3522
stream encryption ciphers, which can be simulated using commonly
3523
available block encryption ciphers, such as the Data Encryption
3524
Standard [DES77], and triple DES variants, in conjunction with block
3525
chaining and checksum methods [DESM80]. Encryption is used to prove the
3526
identities of the network entities participating in message exchanges.
3527
The Key Distribution Center for each realm is trusted by all principals
3528
registered in that realm to store a secret key in confidence. Proof of
3529
knowledge of this secret key is used to verify the authenticity of a
3530
principal.
3531
3532
The KDC uses the principal's secret key (in the AS exchange) or a
3533
shared session key (in the TGS exchange) to encrypt responses to ticket
3534
requests; the ability to obtain the secret key or session key implies
3535
the knowledge of the appropriate keys and the identity of the KDC. The
3536
ability of a principal to decrypt the KDC response and present a Ticket
3537
and a properly formed Authenticator (generated with the session key
3538
from the KDC response) to a service verifies the identity of the
3539
principal; likewise the ability of the service to extract the session
3540
key from the Ticket and prove its knowledge thereof in a response
3541
verifies the identity of the service.
3542
3543
The Kerberos protocols generally assume that the encryption used is
3544
secure from cryptanalysis; however, in some cases, the order of fields
3545
3546
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3547
3548
3549
3550
3551
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3552
3553
in the encrypted portions of messages are arranged to minimize the
3554
effects of poorly chosen keys. It is still important to choose good
3555
keys. If keys are derived from user-typed passwords, those passwords
3556
need to be well chosen to make brute force attacks more difficult.
3557
Poorly chosen keys still make easy targets for intruders.
3558
3559
The following sections specify the encryption and checksum mechanisms
3560
currently defined for Kerberos. The encodings, chaining, and padding
3561
requirements for each are described. For encryption methods, it is
3562
often desirable to place random information (often referred to as a
3563
confounder) at the start of the message. The requirements for a
3564
confounder are specified with each encryption mechanism.
3565
3566
Some encryption systems use a block-chaining method to improve the the
3567
security characteristics of the ciphertext. However, these chaining
3568
methods often don't provide an integrity check upon decryption. Such
3569
systems (such as DES in CBC mode) must be augmented with a checksum of
3570
the plain-text which can be verified at decryption and used to detect
3571
any tampering or damage. Such checksums should be good at detecting
3572
burst errors in the input. If any damage is detected, the decryption
3573
routine is expected to return an error indicating the failure of an
3574
integrity check. Each encryption type is expected to provide and verify
3575
an appropriate checksum. The specification of each encryption method
3576
sets out its checksum requirements.
3577
3578
Finally, where a key is to be derived from a user's password, an
3579
algorithm for converting the password to a key of the appropriate type
3580
is included. It is desirable for the string to key function to be
3581
one-way, and for the mapping to be different in different realms. This
3582
is important because users who are registered in more than one realm
3583
will often use the same password in each, and it is desirable that an
3584
attacker compromising the Kerberos server in one realm not obtain or
3585
derive the user's key in another.
3586
3587
For an discussion of the integrity characteristics of the candidate
3588
encryption and checksum methods considered for Kerberos, the reader is
3589
referred to [SG92].
3590
3591
6.1. Encryption Specifications
3592
3593
The following ASN.1 definition describes all encrypted messages. The
3594
enc-part field which appears in the unencrypted part of messages in
3595
section 5 is a sequence consisting of an encryption type, an optional
3596
key version number, and the ciphertext.
3597
3598
EncryptedData ::= SEQUENCE {
3599
etype[0] INTEGER, -- EncryptionType
3600
kvno[1] INTEGER OPTIONAL,
3601
cipher[2] OCTET STRING -- ciphertext
3602
}
3603
3604
3605
3606
etype
3607
This field identifies which encryption algorithm was used to
3608
encipher the cipher. Detailed specifications for selected
3609
encryption types appear later in this section.
3610
kvno
3611
This field contains the version number of the key under which data
3612
3613
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3614
3615
3616
3617
3618
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3619
3620
is encrypted. It is only present in messages encrypted under long
3621
lasting keys, such as principals' secret keys.
3622
cipher
3623
This field contains the enciphered text, encoded as an OCTET
3624
STRING.
3625
The cipher field is generated by applying the specified encryption
3626
algorithm to data composed of the message and algorithm-specific
3627
inputs. Encryption mechanisms defined for use with Kerberos must take
3628
sufficient measures to guarantee the integrity of the plaintext, and we
3629
recommend they also take measures to protect against precomputed
3630
dictionary attacks. If the encryption algorithm is not itself capable
3631
of doing so, the protections can often be enhanced by adding a checksum
3632
and a confounder.
3633
3634
The suggested format for the data to be encrypted includes a
3635
confounder, a checksum, the encoded plaintext, and any necessary
3636
padding. The msg-seq field contains the part of the protocol message
3637
described in section 5 which is to be encrypted. The confounder,
3638
checksum, and padding are all untagged and untyped, and their length is
3639
exactly sufficient to hold the appropriate item. The type and length is
3640
implicit and specified by the particular encryption type being used
3641
(etype). The format for the data to be encrypted for some methods is
3642
described in the following diagram, but other methods may deviate from
3643
this layour - so long as the definition of the method defines the
3644
layout actually in use.
3645
3646
+-----------+----------+-------------+-----+
3647
|confounder | check | msg-seq | pad |
3648
+-----------+----------+-------------+-----+
3649
3650
The format cannot be described in ASN.1, but for those who prefer an
3651
ASN.1-like notation:
3652
3653
CipherText ::= ENCRYPTED SEQUENCE {
3654
confounder[0] UNTAGGED[35] OCTET STRING(conf_length) OPTIONAL,
3655
check[1] UNTAGGED OCTET STRING(checksum_length) OPTIONAL,
3656
msg-seq[2] MsgSequence,
3657
pad UNTAGGED OCTET STRING(pad_length) OPTIONAL
3658
}
3659
3660
One generates a random confounder of the appropriate length, placing it
3661
in confounder; zeroes out check; calculates the appropriate checksum
3662
over confounder, check, and msg-seq, placing the result in check; adds
3663
the necessary padding; then encrypts using the specified encryption
3664
type and the appropriate key.
3665
3666
Unless otherwise specified, a definition of an encryption algorithm
3667
that specifies a checksum, a length for the confounder field, or an
3668
octet boundary for padding uses this ciphertext format[36]. Those
3669
fields which are not specified will be omitted.
3670
3671
In the interest of allowing all implementations using a particular
3672
encryption type to communicate with all others using that type, the
3673
specification of an encryption type defines any checksum that is needed
3674
as part of the encryption process. If an alternative checksum is to be
3675
used, a new encryption type must be defined.
3676
3677
Some cryptosystems require additional information beyond the key and
3678
the data to be encrypted. For example, DES, when used in
3679
3680
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3681
3682
3683
3684
3685
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3686
3687
cipher-block-chaining mode, requires an initialization vector. If
3688
required, the description for each encryption type must specify the
3689
source of such additional information. 6.2. Encryption Keys
3690
3691
The sequence below shows the encoding of an encryption key:
3692
3693
EncryptionKey ::= SEQUENCE {
3694
keytype[0] INTEGER,
3695
keyvalue[1] OCTET STRING
3696
}
3697
3698
keytype
3699
This field specifies the type of encryption that is to be
3700
performed using the key that follows in the keyvalue field. It
3701
will always correspond to the etype to be used to generate or
3702
decode the EncryptedData. In cases when multiple algorithms use a
3703
common kind of key (e.g., if the encryption algorithm uses an
3704
alternate checksum algorithm for an integrity check, or a
3705
different chaining mechanism), the keytype provides information
3706
needed to determine which algorithm is to be used.
3707
keyvalue
3708
This field contains the key itself, encoded as an octet string.
3709
All negative values for the encryption key type are reserved for local
3710
use. All non-negative values are reserved for officially assigned type
3711
fields and interpreta- tions.
3712
3713
6.3. Encryption Systems
3714
3715
6.3.1. The NULL Encryption System (null)
3716
3717
If no encryption is in use, the encryption system is said to be the
3718
NULL encryption system. In the NULL encryption system there is no
3719
checksum, confounder or padding. The ciphertext is simply the
3720
plaintext. The NULL Key is used by the null encryption system and is
3721
zero octets in length, with keytype zero (0).
3722
3723
6.3.2. DES in CBC mode with a CRC-32 checksum (des-cbc-crc)
3724
3725
The des-cbc-crc encryption mode encrypts information under the Data
3726
Encryption Standard [DES77] using the cipher block chaining mode
3727
[DESM80]. A CRC-32 checksum (described in ISO 3309 [ISO3309]) is
3728
applied to the confounder and message sequence (msg-seq) and placed in
3729
the cksum field. DES blocks are 8 bytes. As a result, the data to be
3730
encrypted (the concatenation of confounder, checksum, and message) must
3731
be padded to an 8 byte boundary before encryption. The details of the
3732
encryption of this data are identical to those for the des-cbc-md5
3733
encryption mode.
3734
3735
Note that, since the CRC-32 checksum is not collision-proof, an
3736
attacker could use a probabilistic chosen-plaintext attack to generate
3737
a valid message even if a confounder is used [SG92]. The use of
3738
collision-proof checksums is recommended for environments where such
3739
attacks represent a significant threat. The use of the CRC-32 as the
3740
checksum for ticket or authenticator is no longer mandated as an
3741
interoperability requirement for Kerberos Version 5 Specification 1
3742
(See section 9.1 for specific details).
3743
3744
6.3.3. DES in CBC mode with an MD4 checksum (des-cbc-md4)
3745
3746
3747
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3748
3749
3750
3751
3752
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3753
3754
The des-cbc-md4 encryption mode encrypts information under the Data
3755
Encryption Standard [DES77] using the cipher block chaining mode
3756
[DESM80]. An MD4 checksum (described in [MD492]) is applied to the
3757
confounder and message sequence (msg-seq) and placed in the cksum
3758
field. DES blocks are 8 bytes. As a result, the data to be encrypted
3759
(the concatenation of confounder, checksum, and message) must be padded
3760
to an 8 byte boundary before encryption. The details of the encryption
3761
of this data are identical to those for the des-cbc-md5 encryption
3762
mode.
3763
3764
6.3.4. DES in CBC mode with an MD5 checksum (des-cbc-md5)
3765
3766
The des-cbc-md5 encryption mode encrypts information under the Data
3767
Encryption Standard [DES77] using the cipher block chaining mode
3768
[DESM80]. An MD5 checksum (described in [MD5-92].) is applied to the
3769
confounder and message sequence (msg-seq) and placed in the cksum
3770
field. DES blocks are 8 bytes. As a result, the data to be encrypted
3771
(the concatenation of confounder, checksum, and message) must be padded
3772
to an 8 byte boundary before encryption.
3773
3774
Plaintext and DES ciphtertext are encoded as blocks of 8 octets which
3775
are concatenated to make the 64-bit inputs for the DES algorithms. The
3776
first octet supplies the 8 most significant bits (with the octet's
3777
MSbit used as the DES input block's MSbit, etc.), the second octet the
3778
next 8 bits, ..., and the eighth octet supplies the 8 least significant
3779
bits.
3780
3781
Encryption under DES using cipher block chaining requires an additional
3782
input in the form of an initialization vector. Unless otherwise
3783
specified, zero should be used as the initialization vector. Kerberos'
3784
use of DES requires an 8 octet confounder.
3785
3786
The DES specifications identify some 'weak' and 'semi-weak' keys; those
3787
keys shall not be used for encrypting messages for use in Kerberos.
3788
Additionally, because of the way that keys are derived for the
3789
encryption of checksums, keys shall not be used that yield 'weak' or
3790
'semi-weak' keys when eXclusive-ORed with the hexadecimal constant
3791
F0F0F0F0F0F0F0F0.
3792
3793
A DES key is 8 octets of data, with keytype one (1). This consists of
3794
56 bits of key, and 8 parity bits (one per octet). The key is encoded
3795
as a series of 8 octets written in MSB-first order. The bits within the
3796
key are also encoded in MSB order. For example, if the encryption key
3797
is (B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8) where
3798
B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the
3799
parity bits, the first octet of the key would be B1,B2,...,B7,P1 (with
3800
B1 as the MSbit). [See the FIPS 81 introduction for reference.]
3801
3802
String to key transformation
3803
3804
To generate a DES key from a text string (password), a "salt" is
3805
concatenated to the text string, and then padded with ASCII nulls to an
3806
8 byte boundary. This "salt" is normally the realm and each component
3807
of the principal's name appended. However, sometimes different salts
3808
are used --- for example, when a realm is renamed, or if a user changes
3809
her username, or for compatibility with Kerberos V4 (whose
3810
string-to-key algorithm uses a null string for the salt). This string
3811
is then fan-folded and eXclusive-ORed with itself to form an 8 byte DES
3812
key. Before eXclusive-ORing a block, every byte is shifted one bit to
3813
3814
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3815
3816
3817
3818
3819
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3820
3821
the left to leave the lowest bit zero. The key is the "corrected" by
3822
correcting the parity on the key, and if the key matches a 'weak' or
3823
'semi-weak' key as described in the DES specification, it is
3824
eXclusive-ORed with the constant 00000000000000F0. This key is then
3825
used to generate a DES CBC checksum on the initial string (with the
3826
salt appended). The result of the CBC checksum is the "corrected" as
3827
described above to form the result which is return as the key.
3828
Pseudocode follows:
3829
3830
name_to_default_salt(realm, name) {
3831
s = realm
3832
for(each component in name) {
3833
s = s + component;
3834
}
3835
return s;
3836
}
3837
3838
key_correction(key) {
3839
fixparity(key);
3840
if (is_weak_key_key(key))
3841
key = key XOR 0xF0;
3842
return(key);
3843
}
3844
3845
string_to_key(string,salt) {
3846
3847
odd = 1;
3848
s = string + salt;
3849
tempkey = NULL;
3850
pad(s); /* with nulls to 8 byte boundary */
3851
for(8byteblock in s) {
3852
if(odd == 0) {
3853
odd = 1;
3854
reverse(8byteblock)
3855
}
3856
else odd = 0;
3857
left shift every byte in 8byteblock one bit;
3858
tempkey = tempkey XOR 8byteblock;
3859
}
3860
tempkey = key_correction(tempkey);
3861
key = key_correction(DES-CBC-check(s,tempkey));
3862
return(key);
3863
}
3864
3865
6.3.5. Triple DES with HMAC-SHA1 Kerberos Encryption Type with and
3866
without Key Derivation [Original draft by Marc Horowitz, revisions by
3867
David Miller]
3868
3869
This encryption type is based on the Triple DES cryptosystem, the
3870
HMAC-SHA1 [Krawczyk96] message authentication algorithm, and key
3871
derivation for Kerberos V5 [HorowitzB96]. Key derivation may or may not
3872
be used in conjunction with the use of Triple DES keys.
3873
3874
Algorithm Identifiers
3875
3876
The des3-cbc-hmac-sha1 encryption type has been assigned the value 7.
3877
The des3-cbc-hmac-sha1-kd encryption type, specifying the key
3878
derivation variant of the encryption type, has been assigned the value
3879
16. The hmac-sha1-des3 checksum type has been assigned the value 13.
3880
3881
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3882
3883
3884
3885
3886
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3887
3888
The hmac-sha1-des3-kd checksum type, specifying the key derivation
3889
variant of the checksum, has been assigned the value 12.
3890
3891
Triple DES Key Production
3892
3893
The EncryptionKey value is 24 octets long. The 7 most significant bits
3894
of each octet contain key bits, and the least significant bit is the
3895
inverse of the xor of the key bits.
3896
3897
For the purposes of key derivation, the block size is 64 bits, and the
3898
key size is 168 bits. The 168 bits output by key derivation are
3899
converted to an EncryptionKey value as follows. First, the 168 bits are
3900
divided into three groups of 56 bits, which are expanded individually
3901
into 64 bits as follows:
3902
3903
1 2 3 4 5 6 7 p
3904
9 10 11 12 13 14 15 p
3905
17 18 19 20 21 22 23 p
3906
25 26 27 28 29 30 31 p
3907
33 34 35 36 37 38 39 p
3908
41 42 43 44 45 46 47 p
3909
49 50 51 52 53 54 55 p
3910
56 48 40 32 24 16 8 p
3911
3912
The "p" bits are parity bits computed over the data bits. The output of
3913
the three expansions are concatenated to form the EncryptionKey value.
3914
3915
When the HMAC-SHA1 of a string is computed, the key is used in the
3916
EncryptedKey form.
3917
3918
The string-to-key function is used to tranform UNICODE passwords into
3919
DES3 keys. The DES3 string-to-key function relies on the "N-fold"
3920
algorithm, which is detailed in [9]. The description of the N-fold
3921
algorithm in that document is as follows:
3922
o To n-fold a number X, replicate the input value to a length that
3923
is the least common multiple of n and the length of X. Before each
3924
repetition, the input is rotated to the right by 13 bit positions.
3925
The successive n-bit chunks are added together using
3926
1's-complement addition (that is, addition with end-around carry)
3927
to yield an n-bit result"
3928
o The n-fold algorithm, as with DES string-to-key, is applied to the
3929
password string concatenated with a salt value. The salt value is
3930
derived in the same was as for the DES string-to-key algorithm.
3931
For 3-key triple DES then, the operation will involve a 168-fold
3932
of the input password string. The remainder of the string-to-key
3933
function for DES3 is shown here in pseudocode:
3934
3935
DES3string-to-key(passwordString, key)
3936
3937
salt = name_to_default_salt(realm, name)
3938
s = passwordString + salt
3939
tmpKey1 = 168-fold(s)
3940
parityFix(tmpKey1);
3941
if not weakKey(tmpKey1)
3942
/*
3943
* Encrypt temp key in itself with a
3944
* zero initialization vector
3945
*
3946
* Function signature is DES3encrypt(plain, key, iv)
3947
3948
Neuman, Ts'o, Kohl Expires: 10 September, 2000
3949
3950
3951
3952
3953
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
3954
3955
* with cipher as the return value
3956
*/
3957
tmpKey2 = DES3encrypt(tmpKey1, tmpKey1, zeroIvec)
3958
/*
3959
* Encrypt resultant temp key in itself with third component
3960
* of first temp key as initialization vector
3961
*/
3962
key = DES3encrypt(tmpKey2, tmpKey1, tmpKey1[2])
3963
parityFix(key)
3964
if not weakKey(key)
3965
return SUCCESS
3966
else
3967
return FAILURE
3968
else
3969
return FAILURE
3970
3971
The weakKey function above is the same weakKey function used with DES
3972
keys, but applied to each of the three single DES keys that comprise
3973
the triple DES key.
3974
3975
The lengths of UNICODE encoded character strings include the trailing
3976
terminator character (0).
3977
3978
Encryption Types des3-cbc-hmac-sha1 and des3-cbc-hmac-sha1-kd
3979
3980
EncryptedData using this type must be generated as described in
3981
[Horowitz96]. The encryption algorithm is Triple DES in Outer-CBC mode.
3982
The checksum algorithm is HMAC-SHA1. If the key derivation variant of
3983
the encryption type is used, encryption key values are modified
3984
according to the method under the Key Derivation section below.
3985
3986
Unless otherwise specified, a zero IV must be used.
3987
3988
If the length of the input data is not a multiple of the block size,
3989
zero octets must be used to pad the plaintext to the next eight-octet
3990
boundary. The counfounder must be eight random octets (one block).
3991
3992
Checksum Types hmac-sha1-des3 and hmac-sha1-des3-kd
3993
3994
Checksums using this type must be generated as described in
3995
[Horowitz96]. The keyed hash algorithm is HMAC-SHA1. If the key
3996
derivation variant of the checksum type is used, checksum key values
3997
are modified according to the method under the Key Derivation section
3998
below.
3999
4000
Key Derivation
4001
4002
In the Kerberos protocol, cryptographic keys are used in a number of
4003
places. In order to minimize the effect of compromising a key, it is
4004
desirable to use a different key for each of these places. Key
4005
derivation [Horowitz96] can be used to construct different keys for
4006
each operation from the keys transported on the network. For this to be
4007
possible, a small change to the specification is necessary.
4008
4009
This section specifies a profile for the use of key derivation
4010
[Horowitz96] with Kerberos. For each place where a key is used, a ``key
4011
usage'' must is specified for that purpose. The key, key usage, and
4012
encryption/checksum type together describe the transformation from
4013
plaintext to ciphertext, or plaintext to checksum.
4014
4015
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4016
4017
4018
4019
4020
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4021
4022
4023
Key Usage Values
4024
4025
This is a complete list of places keys are used in the kerberos
4026
protocol, with key usage values and RFC 1510 section numbers:
4027
4028
1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the
4029
client key (section 5.4.1)
4030
2. AS-REP Ticket and TGS-REP Ticket (includes tgs session key or
4031
application session key), encrypted with the service key
4032
(section 5.4.2)
4033
3. AS-REP encrypted part (includes tgs session key or application
4034
session key), encrypted with the client key (section 5.4.2)
4035
4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs
4036
session key (section 5.4.1)
4037
5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs
4038
authenticator subkey (section 5.4.1)
4039
6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed
4040
with the tgs session key (sections 5.3.2, 5.4.1)
4041
7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes tgs
4042
authenticator subkey), encrypted with the tgs session key
4043
(section 5.3.2)
4044
8. TGS-REP encrypted part (includes application session key),
4045
encrypted with the tgs session key (section 5.4.2)
4046
9. TGS-REP encrypted part (includes application session key),
4047
encrypted with the tgs authenticator subkey (section 5.4.2)
4048
10. AP-REQ Authenticator cksum, keyed with the application session
4049
key (section 5.3.2)
4050
11. AP-REQ Authenticator (includes application authenticator
4051
subkey), encrypted with the application session key (section
4052
5.3.2)
4053
12. AP-REP encrypted part (includes application session subkey),
4054
encrypted with the application session key (section 5.5.2)
4055
13. KRB-PRIV encrypted part, encrypted with a key chosen by the
4056
application (section 5.7.1)
4057
14. KRB-CRED encrypted part, encrypted with a key chosen by the
4058
application (section 5.6.1)
4059
15. KRB-SAVE cksum, keyed with a key chosen by the application
4060
(section 5.8.1)
4061
18. KRB-ERROR checksum (e-cksum in section 5.9.1)
4062
19. AD-KDCIssued checksum (ad-checksum in appendix B.1)
4063
20. Checksum for Mandatory Ticket Extensions (appendix B.6)
4064
21. Checksum in Authorization Data in Ticket Extensions (appendix B.7)
4065
4066
Key usage values between 1024 and 2047 (inclusive) are reserved for
4067
application use. Applications should use even values for encryption and
4068
odd values for checksums within this range.
4069
4070
A few of these key usages need a little clarification. A service which
4071
receives an AP-REQ has no way to know if the enclosed Ticket was part
4072
of an AS-REP or TGS-REP. Therefore, key usage 2 must always be used for
4073
generating a Ticket, whether it is in response to an AS- REQ or
4074
TGS-REQ.
4075
4076
There might exist other documents which define protocols in terms of
4077
the RFC1510 encryption types or checksum types. Such documents would
4078
not know about key usages. In order that these documents continue to be
4079
meaningful until they are updated, key usages 1024 and 1025 must be
4080
used to derive keys for encryption and checksums, respectively. New
4081
4082
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4083
4084
4085
4086
4087
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4088
4089
protocols defined in terms of the Kerberos encryption and checksum
4090
types should use their own key usages. Key usages may be registered
4091
with IANA to avoid conflicts. Key usages must be unsigned 32 bit
4092
integers. Zero is not permitted.
4093
4094
Defining Cryptosystems Using Key Derivation
4095
4096
Kerberos requires that the ciphertext component of EncryptedData be
4097
tamper-resistant as well as confidential. This implies encryption and
4098
integrity functions, which must each use their own separate keys. So,
4099
for each key usage, two keys must be generated, one for encryption
4100
(Ke), and one for integrity (Ki):
4101
4102
Ke = DK(protocol key, key usage | 0xAA)
4103
Ki = DK(protocol key, key usage | 0x55)
4104
4105
where the protocol key is from the EncryptionKey from the wire
4106
protocol, and the key usage is represented as a 32 bit integer in
4107
network byte order. The ciphertest must be generated from the plaintext
4108
as follows:
4109
4110
ciphertext = E(Ke, confounder | plaintext | padding) |
4111
H(Ki, confounder | plaintext | padding)
4112
4113
The confounder and padding are specific to the encryption algorithm E.
4114
4115
When generating a checksum only, there is no need for a confounder or
4116
padding. Again, a new key (Kc) must be used. Checksums must be
4117
generated from the plaintext as follows:
4118
4119
Kc = DK(protocol key, key usage | 0x99)
4120
MAC = H(Kc, plaintext)
4121
4122
Note that each enctype is described by an encryption algorithm E and a
4123
keyed hash algorithm H, and each checksum type is described by a keyed
4124
hash algorithm H. HMAC, with an appropriate hash, is required for use
4125
as H.
4126
4127
Key Derivation from Passwords
4128
4129
The well-known constant for password key derivation must be the byte
4130
string {0x6b 0x65 0x72 0x62 0x65 0x72 0x6f 0x73}. These values
4131
correspond to the ASCII encoding for the string "kerberos".
4132
4133
6.4. Checksums
4134
4135
The following is the ASN.1 definition used for a checksum:
4136
4137
Checksum ::= SEQUENCE {
4138
cksumtype[0] INTEGER,
4139
checksum[1] OCTET STRING
4140
}
4141
4142
cksumtype
4143
This field indicates the algorithm used to generate the
4144
accompanying checksum.
4145
checksum
4146
This field contains the checksum itself, encoded as an octet
4147
string.
4148
4149
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4150
4151
4152
4153
4154
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4155
4156
Detailed specification of selected checksum types appear later in this
4157
section. Negative values for the checksum type are reserved for local
4158
use. All non-negative values are reserved for officially assigned type
4159
fields and interpretations.
4160
4161
Checksums used by Kerberos can be classified by two properties: whether
4162
they are collision-proof, and whether they are keyed. It is infeasible
4163
to find two plaintexts which generate the same checksum value for a
4164
collision-proof checksum. A key is required to perturb or initialize
4165
the algorithm in a keyed checksum. To prevent message-stream
4166
modification by an active attacker, unkeyed checksums should only be
4167
used when the checksum and message will be subsequently encrypted (e.g.
4168
the checksums defined as part of the encryption algorithms covered
4169
earlier in this section).
4170
4171
Collision-proof checksums can be made tamper-proof if the checksum
4172
value is encrypted before inclusion in a message. In such cases, the
4173
composition of the checksum and the encryption algorithm must be
4174
considered a separate checksum algorithm (e.g. RSA-MD5 encrypted using
4175
DES is a new checksum algorithm of type RSA-MD5-DES). For most keyed
4176
checksums, as well as for the encrypted forms of unkeyed
4177
collision-proof checksums, Kerberos prepends a confounder before the
4178
checksum is calculated.
4179
4180
6.4.1. The CRC-32 Checksum (crc32)
4181
4182
The CRC-32 checksum calculates a checksum based on a cyclic redundancy
4183
check as described in ISO 3309 [ISO3309]. The resulting checksum is
4184
four (4) octets in length. The CRC-32 is neither keyed nor
4185
collision-proof. The use of this checksum is not recommended. An
4186
attacker using a probabilistic chosen-plaintext attack as described in
4187
[SG92] might be able to generate an alternative message that satisfies
4188
the checksum. The use of collision-proof checksums is recommended for
4189
environments where such attacks represent a significant threat.
4190
4191
6.4.2. The RSA MD4 Checksum (rsa-md4)
4192
4193
The RSA-MD4 checksum calculates a checksum using the RSA MD4 algorithm
4194
[MD4-92]. The algorithm takes as input an input message of arbitrary
4195
length and produces as output a 128-bit (16 octet) checksum. RSA-MD4 is
4196
believed to be collision-proof.
4197
4198
6.4.3. RSA MD4 Cryptographic Checksum Using DES (rsa-md4-des)
4199
4200
The RSA-MD4-DES checksum calculates a keyed collision-proof checksum by
4201
prepending an 8 octet confounder before the text, applying the RSA MD4
4202
checksum algorithm, and encrypting the confounder and the checksum
4203
using DES in cipher-block-chaining (CBC) mode using a variant of the
4204
key, where the variant is computed by eXclusive-ORing the key with the
4205
constant F0F0F0F0F0F0F0F0[39]. The initialization vector should be
4206
zero. The resulting checksum is 24 octets long (8 octets of which are
4207
redundant). This checksum is tamper-proof and believed to be
4208
collision-proof.
4209
4210
The DES specifications identify some weak keys' and 'semi-weak keys';
4211
those keys shall not be used for generating RSA-MD4 checksums for use
4212
in Kerberos.
4213
4214
The format for the checksum is described in the follow- ing diagram:
4215
4216
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4217
4218
4219
4220
4221
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4222
4223
4224
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
4225
| des-cbc(confounder + rsa-md4(confounder+msg),key=var(key),iv=0) |
4226
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
4227
4228
The format cannot be described in ASN.1, but for those who prefer an
4229
ASN.1-like notation:
4230
4231
rsa-md4-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
4232
confounder[0] UNTAGGED OCTET STRING(8),
4233
check[1] UNTAGGED OCTET STRING(16)
4234
}
4235
4236
6.4.4. The RSA MD5 Checksum (rsa-md5)
4237
4238
The RSA-MD5 checksum calculates a checksum using the RSA MD5 algorithm.
4239
[MD5-92]. The algorithm takes as input an input message of arbitrary
4240
length and produces as output a 128-bit (16 octet) checksum. RSA-MD5 is
4241
believed to be collision-proof.
4242
4243
6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5-des)
4244
4245
The RSA-MD5-DES checksum calculates a keyed collision-proof checksum by
4246
prepending an 8 octet confounder before the text, applying the RSA MD5
4247
checksum algorithm, and encrypting the confounder and the checksum
4248
using DES in cipher-block-chaining (CBC) mode using a variant of the
4249
key, where the variant is computed by eXclusive-ORing the key with the
4250
hexadecimal constant F0F0F0F0F0F0F0F0. The initialization vector should
4251
be zero. The resulting checksum is 24 octets long (8 octets of which
4252
are redundant). This checksum is tamper-proof and believed to be
4253
collision-proof.
4254
4255
The DES specifications identify some 'weak keys' and 'semi-weak keys';
4256
those keys shall not be used for encrypting RSA-MD5 checksums for use
4257
in Kerberos.
4258
4259
The format for the checksum is described in the following diagram:
4260
4261
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
4262
| des-cbc(confounder + rsa-md5(confounder+msg),key=var(key),iv=0) |
4263
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
4264
4265
The format cannot be described in ASN.1, but for those who prefer an
4266
ASN.1-like notation:
4267
4268
rsa-md5-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
4269
confounder[0] UNTAGGED OCTET STRING(8),
4270
check[1] UNTAGGED OCTET STRING(16)
4271
}
4272
4273
6.4.6. DES cipher-block chained checksum (des-mac)
4274
4275
The DES-MAC checksum is computed by prepending an 8 octet confounder to
4276
the plaintext, performing a DES CBC-mode encryption on the result using
4277
the key and an initialization vector of zero, taking the last block of
4278
the ciphertext, prepending the same confounder and encrypting the pair
4279
using DES in cipher-block-chaining (CBC) mode using a a variant of the
4280
key, where the variant is computed by eXclusive-ORing the key with the
4281
hexadecimal constant F0F0F0F0F0F0F0F0. The initialization vector should
4282
4283
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4284
4285
4286
4287
4288
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4289
4290
be zero. The resulting checksum is 128 bits (16 octets) long, 64 bits
4291
of which are redundant. This checksum is tamper-proof and
4292
collision-proof.
4293
4294
The format for the checksum is described in the following diagram:
4295
4296
+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+
4297
| des-cbc(confounder + des-mac(conf+msg,iv=0,key),key=var(key),iv=0) |
4298
+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+
4299
4300
The format cannot be described in ASN.1, but for those who prefer an
4301
ASN.1-like notation:
4302
4303
des-mac-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
4304
confounder[0] UNTAGGED OCTET STRING(8),
4305
check[1] UNTAGGED OCTET STRING(8)
4306
}
4307
4308
The DES specifications identify some 'weak' and 'semi-weak' keys; those
4309
keys shall not be used for generating DES-MAC checksums for use in
4310
Kerberos, nor shall a key be used whose variant is 'weak' or
4311
'semi-weak'.
4312
4313
6.4.7. RSA MD4 Cryptographic Checksum Using DES alternative
4314
(rsa-md4-des-k)
4315
4316
The RSA-MD4-DES-K checksum calculates a keyed collision-proof checksum
4317
by applying the RSA MD4 checksum algorithm and encrypting the results
4318
using DES in cipher-block-chaining (CBC) mode using a DES key as both
4319
key and initialization vector. The resulting checksum is 16 octets
4320
long. This checksum is tamper-proof and believed to be collision-proof.
4321
Note that this checksum type is the old method for encoding the
4322
RSA-MD4-DES checksum and it is no longer recommended.
4323
4324
6.4.8. DES cipher-block chained checksum alternative (des-mac-k)
4325
4326
The DES-MAC-K checksum is computed by performing a DES CBC-mode
4327
encryption of the plaintext, and using the last block of the ciphertext
4328
as the checksum value. It is keyed with an encryption key and an
4329
initialization vector; any uses which do not specify an additional
4330
initialization vector will use the key as both key and initialization
4331
vector. The resulting checksum is 64 bits (8 octets) long. This
4332
checksum is tamper-proof and collision-proof. Note that this checksum
4333
type is the old method for encoding the DES-MAC checksum and it is no
4334
longer recommended. The DES specifications identify some 'weak keys'
4335
and 'semi-weak keys'; those keys shall not be used for generating
4336
DES-MAC checksums for use in Kerberos.
4337
4338
7. Naming Constraints
4339
4340
7.1. Realm Names
4341
4342
Although realm names are encoded as GeneralStrings and although a realm
4343
can technically select any name it chooses, interoperability across
4344
realm boundaries requires agreement on how realm names are to be
4345
assigned, and what information they imply.
4346
4347
To enforce these conventions, each realm must conform to the
4348
conventions itself, and it must require that any realms with which
4349
4350
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4351
4352
4353
4354
4355
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4356
4357
inter-realm keys are shared also conform to the conventions and require
4358
the same from its neighbors.
4359
4360
Kerberos realm names are case sensitive. Realm names that differ only
4361
in the case of the characters are not equivalent. There are presently
4362
four styles of realm names: domain, X500, other, and reserved. Examples
4363
of each style follow:
4364
4365
domain: ATHENA.MIT.EDU (example)
4366
X500: C=US/O=OSF (example)
4367
other: NAMETYPE:rest/of.name=without-restrictions (example)
4368
reserved: reserved, but will not conflict with above
4369
4370
Domain names must look like domain names: they consist of components
4371
separated by periods (.) and they contain neither colons (:) nor
4372
slashes (/). Domain names must be converted to upper case when used as
4373
realm names.
4374
4375
X.500 names contain an equal (=) and cannot contain a colon (:) before
4376
the equal. The realm names for X.500 names will be string
4377
representations of the names with components separated by slashes.
4378
Leading and trailing slashes will not be included.
4379
4380
Names that fall into the other category must begin with a prefix that
4381
contains no equal (=) or period (.) and the prefix must be followed by
4382
a colon (:) and the rest of the name. All prefixes must be assigned
4383
before they may be used. Presently none are assigned.
4384
4385
The reserved category includes strings which do not fall into the first
4386
three categories. All names in this category are reserved. It is
4387
unlikely that names will be assigned to this category unless there is a
4388
very strong argument for not using the 'other' category.
4389
4390
These rules guarantee that there will be no conflicts between the
4391
various name styles. The following additional constraints apply to the
4392
assignment of realm names in the domain and X.500 categories: the name
4393
of a realm for the domain or X.500 formats must either be used by the
4394
organization owning (to whom it was assigned) an Internet domain name
4395
or X.500 name, or in the case that no such names are registered,
4396
authority to use a realm name may be derived from the authority of the
4397
parent realm. For example, if there is no domain name for E40.MIT.EDU,
4398
then the administrator of the MIT.EDU realm can authorize the creation
4399
of a realm with that name.
4400
4401
This is acceptable because the organization to which the parent is
4402
assigned is presumably the organization authorized to assign names to
4403
its children in the X.500 and domain name systems as well. If the
4404
parent assigns a realm name without also registering it in the domain
4405
name or X.500 hierarchy, it is the parent's responsibility to make sure
4406
that there will not in the future exists a name identical to the realm
4407
name of the child unless it is assigned to the same entity as the realm
4408
name.
4409
4410
7.2. Principal Names
4411
4412
As was the case for realm names, conventions are needed to ensure that
4413
all agree on what information is implied by a principal name. The
4414
name-type field that is part of the principal name indicates the kind
4415
of information implied by the name. The name-type should be treated as
4416
4417
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4418
4419
4420
4421
4422
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4423
4424
a hint. Ignoring the name type, no two names can be the same (i.e. at
4425
least one of the components, or the realm, must be different). The
4426
following name types are defined:
4427
4428
name-type value meaning
4429
4430
NT-UNKNOWN 0 Name type not known
4431
NT-PRINCIPAL 1 General principal name (e.g. username, or DCE principal)
4432
NT-SRV-INST 2 Service and other unique instance (krbtgt)
4433
NT-SRV-HST 3 Service with host name as instance (telnet, rcommands)
4434
NT-SRV-XHST 4 Service with slash-separated host name components
4435
NT-UID 5 Unique ID
4436
NT-X500-PRINCIPAL 6 Encoded X.509 Distingished name [RFC 1779]
4437
4438
When a name implies no information other than its uniqueness at a
4439
particular time the name type PRINCIPAL should be used. The principal
4440
name type should be used for users, and it might also be used for a
4441
unique server. If the name is a unique machine generated ID that is
4442
guaranteed never to be reassigned then the name type of UID should be
4443
used (note that it is generally a bad idea to reassign names of any
4444
type since stale entries might remain in access control lists).
4445
4446
If the first component of a name identifies a service and the remaining
4447
components identify an instance of the service in a server specified
4448
manner, then the name type of SRV-INST should be used. An example of
4449
this name type is the Kerberos ticket-granting service whose name has a
4450
first component of krbtgt and a second component identifying the realm
4451
for which the ticket is valid.
4452
4453
If instance is a single component following the service name and the
4454
instance identifies the host on which the server is running, then the
4455
name type SRV-HST should be used. This type is typically used for
4456
Internet services such as telnet and the Berkeley R commands. If the
4457
separate components of the host name appear as successive components
4458
following the name of the service, then the name type SRV-XHST should
4459
be used. This type might be used to identify servers on hosts with
4460
X.500 names where the slash (/) might otherwise be ambiguous.
4461
4462
A name type of NT-X500-PRINCIPAL should be used when a name from an
4463
X.509 certificiate is translated into a Kerberos name. The encoding of
4464
the X.509 name as a Kerberos principal shall conform to the encoding
4465
rules specified in RFC 2253.
4466
4467
A name type of UNKNOWN should be used when the form of the name is not
4468
known. When comparing names, a name of type UNKNOWN will match
4469
principals authenticated with names of any type. A principal
4470
authenticated with a name of type UNKNOWN, however, will only match
4471
other names of type UNKNOWN.
4472
4473
Names of any type with an initial component of 'krbtgt' are reserved
4474
for the Kerberos ticket granting service. See section 8.2.3 for the
4475
form of such names.
4476
4477
7.2.1. Name of server principals
4478
4479
The principal identifier for a server on a host will generally be
4480
composed of two parts: (1) the realm of the KDC with which the server
4481
is registered, and (2) a two-component name of type NT-SRV-HST if the
4482
host name is an Internet domain name or a multi-component name of type
4483
4484
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4485
4486
4487
4488
4489
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4490
4491
NT-SRV-XHST if the name of the host is of a form such as X.500 that
4492
allows slash (/) separators. The first component of the two- or
4493
multi-component name will identify the service and the latter
4494
components will identify the host. Where the name of the host is not
4495
case sensitive (for example, with Internet domain names) the name of
4496
the host must be lower case. If specified by the application protocol
4497
for services such as telnet and the Berkeley R commands which run with
4498
system privileges, the first component may be the string 'host' instead
4499
of a service specific identifier. When a host has an official name and
4500
one or more aliases, the official name of the host must be used when
4501
constructing the name of the server principal.
4502
4503
8. Constants and other defined values
4504
4505
8.1. Host address types
4506
4507
All negative values for the host address type are reserved for local
4508
use. All non-negative values are reserved for officially assigned type
4509
fields and interpretations.
4510
4511
The values of the types for the following addresses are chosen to match
4512
the defined address family constants in the Berkeley Standard
4513
Distributions of Unix. They can be found in with symbolic names AF_xxx
4514
(where xxx is an abbreviation of the address family name).
4515
4516
Internet (IPv4) Addresses
4517
4518
Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded in
4519
MSB order. The type of IPv4 addresses is two (2).
4520
4521
Internet (IPv6) Addresses [Westerlund]
4522
4523
IPv6 addresses are 128-bit (16-octet) quantities, encoded in MSB order.
4524
The type of IPv6 addresses is twenty-four (24). [RFC1883] [RFC1884].
4525
The following addresses (see [RFC1884]) MUST not appear in any Kerberos
4526
packet:
4527
o the Unspecified Address
4528
o the Loopback Address
4529
o Link-Local addresses
4530
IPv4-mapped IPv6 addresses MUST be represented as addresses of type 2.
4531
4532
CHAOSnet addresses
4533
4534
CHAOSnet addresses are 16-bit (2-octet) quantities, encoded in MSB
4535
order. The type of CHAOSnet addresses is five (5).
4536
4537
ISO addresses
4538
4539
ISO addresses are variable-length. The type of ISO addresses is seven
4540
(7).
4541
4542
Xerox Network Services (XNS) addresses
4543
4544
XNS addresses are 48-bit (6-octet) quantities, encoded in MSB order.
4545
The type of XNS addresses is six (6).
4546
4547
AppleTalk Datagram Delivery Protocol (DDP) addresses
4548
4549
AppleTalk DDP addresses consist of an 8-bit node number and a 16-bit
4550
4551
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4552
4553
4554
4555
4556
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4557
4558
network number. The first octet of the address is the node number; the
4559
remaining two octets encode the network number in MSB order. The type
4560
of AppleTalk DDP addresses is sixteen (16).
4561
4562
DECnet Phase IV addresses
4563
4564
DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order.
4565
The type of DECnet Phase IV addresses is twelve (12).
4566
4567
Netbios addresses
4568
4569
Netbios addresses are 16-octet addresses typically composed of 1 to 15
4570
characters, trailing blank (ascii char 20) filled, with a 16th octet of
4571
0x0. The type of Netbios addresses is 20 (0x14).
4572
4573
8.2. KDC messages
4574
4575
8.2.1. UDP/IP transport
4576
4577
When contacting a Kerberos server (KDC) for a KRB_KDC_REQ request using
4578
UDP IP transport, the client shall send a UDP datagram containing only
4579
an encoding of the request to port 88 (decimal) at the KDC's IP
4580
address; the KDC will respond with a reply datagram containing only an
4581
encoding of the reply message (either a KRB_ERROR or a KRB_KDC_REP) to
4582
the sending port at the sender's IP address. Kerberos servers
4583
supporting IP transport must accept UDP requests on port 88 (decimal).
4584
The response to a request made through UDP/IP transport must also use
4585
UDP/IP transport.
4586
4587
8.2.2. TCP/IP transport [Westerlund,Danielsson]
4588
4589
Kerberos servers (KDC's) should accept TCP requests on port 88
4590
(decimal) and clients should support the sending of TCP requests on
4591
port 88 (decimal). When the KRB_KDC_REQ message is sent to the KDC over
4592
a TCP stream, a new connection will be established for each
4593
authentication exchange (request and response). The KRB_KDC_REP or
4594
KRB_ERROR message will be returned to the client on the same TCP stream
4595
that was established for the request. The response to a request made
4596
through TCP/IP transport must also use TCP/IP transport. Implementors
4597
should note that some extentions to the Kerberos protocol will not work
4598
if any implementation not supporting the TCP transport is involved
4599
(client or KDC). Implementors are strongly urged to support the TCP
4600
transport on both the client and server and are advised that the
4601
current notation of "should" support will likely change in the future
4602
to must support. The KDC may close the TCP stream after sending a
4603
response, but may leave the stream open if it expects a followup - in
4604
which case it may close the stream at any time if resource constratints
4605
or other factors make it desirable to do so. Care must be taken in
4606
managing TCP/IP connections with the KDC to prevent denial of service
4607
attacks based on the number of TCP/IP connections with the KDC that
4608
remain open. If multiple exchanges with the KDC are needed for certain
4609
forms of preauthentication, multiple TCP connections may be required. A
4610
client may close the stream after receiving response, and should close
4611
the stream if it does not expect to send followup messages. The client
4612
must be prepared to have the stream closed by the KDC at anytime, in
4613
which case it must simply connect again when it is ready to send
4614
subsequent messages.
4615
4616
The first four octets of the TCP stream used to transmit the request
4617
4618
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4619
4620
4621
4622
4623
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4624
4625
request will encode in network byte order the length of the request
4626
(KRB_KDC_REQ), and the length will be followed by the request itself.
4627
The response will similarly be preceeded by a 4 octet encoding in
4628
network byte order of the length of the KRB_KDC_REP or the KRB_ERROR
4629
message and will be followed by the KRB_KDC_REP or the KRB_ERROR
4630
response. If the sign bit is set on the integer represented by the
4631
first 4 octets, then the next 4 octets will be read, extending the
4632
length of the field by another 4 octets (less the sign bit which is
4633
reserved for future expansion).
4634
4635
8.2.3. OSI transport
4636
4637
During authentication of an OSI client to an OSI server, the mutual
4638
authentication of an OSI server to an OSI client, the transfer of
4639
credentials from an OSI client to an OSI server, or during exchange of
4640
private or integrity checked messages, Kerberos protocol messages may
4641
be treated as opaque objects and the type of the authentication
4642
mechanism will be:
4643
4644
OBJECT IDENTIFIER ::= {iso (1), org(3), dod(6),internet(1), security(5),kerberosv5(2)}
4645
4646
Depending on the situation, the opaque object will be an authentication
4647
header (KRB_AP_REQ), an authentication reply (KRB_AP_REP), a safe
4648
message (KRB_SAFE), a private message (KRB_PRIV), or a credentials
4649
message (KRB_CRED). The opaque data contains an application code as
4650
specified in the ASN.1 description for each message. The application
4651
code may be used by Kerberos to determine the message type.
4652
4653
8.2.3. Name of the TGS
4654
4655
The principal identifier of the ticket-granting service shall be
4656
composed of three parts: (1) the realm of the KDC issuing the TGS
4657
ticket (2) a two-part name of type NT-SRV-INST, with the first part
4658
"krbtgt" and the second part the name of the realm which will accept
4659
the ticket-granting ticket. For example, a ticket-granting ticket
4660
issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
4661
ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU"
4662
(realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A ticket-granting ticket
4663
issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
4664
MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU" (realm),
4665
("krbtgt", "MIT.EDU") (name).
4666
4667
8.3. Protocol constants and associated values
4668
4669
The following tables list constants used in the protocol and defines
4670
their meanings. Ranges are specified in the "specification" section
4671
that limit the values of constants for which values are defined here.
4672
This allows implementations to make assumptions about the maximum
4673
values that will be received for these constants. Implementation
4674
receiving values outside the range specified in the "specification"
4675
section may reject the request, but they must recover cleanly.
4676
4677
Encryption type etype value block size minimum pad size confounder size
4678
NULL 0 1 0 0
4679
des-cbc-crc 1 8 4 8
4680
des-cbc-md4 2 8 0 8
4681
des-cbc-md5 3 8 0 8
4682
<reserved> 4
4683
des3-cbc-md5 5 8 0 8
4684
4685
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4686
4687
4688
4689
4690
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4691
4692
<reserved> 6
4693
des3-cbc-sha1 7 8 0 8
4694
dsaWithSHA1-CmsOID 9 (pkinit)
4695
md5WithRSAEncryption-CmsOID 10 (pkinit)
4696
sha1WithRSAEncryption-CmsOID 11 (pkinit)
4697
rc2CBC-EnvOID 12 (pkinit)
4698
rsaEncryption-EnvOID 13 (pkinit from PKCS#1 v1.5)
4699
rsaES-OAEP-ENV-OID 14 (pkinit from PKCS#1 v2.0)
4700
des-ede3-cbc-Env-OID 15 (pkinit)
4701
des3-cbc-sha1-kd 16 (Tom Yu)
4702
rc4-hmac 23 (swift)
4703
rc4-hmac-exp 24 (swift)
4704
4705
ENCTYPE_PK_CROSS 48 (reserved for pkcross)
4706
<reserved> 0x8003
4707
4708
Checksum type sumtype value checksum size
4709
CRC32 1 4
4710
rsa-md4 2 16
4711
rsa-md4-des 3 24
4712
des-mac 4 16
4713
des-mac-k 5 8
4714
rsa-md4-des-k 6 16 (drop rsa ?)
4715
rsa-md5 7 16 (drop rsa ?)
4716
rsa-md5-des 8 24 (drop rsa ?)
4717
rsa-md5-des3 9 24 (drop rsa ?)
4718
hmac-sha1-des3-kd 12 20
4719
hmac-sha1-des3 13 20
4720
4721
padata type padata-type value
4722
4723
PA-TGS-REQ 1
4724
PA-ENC-TIMESTAMP 2
4725
PA-PW-SALT 3
4726
<reserved> 4
4727
PA-ENC-UNIX-TIME 5 (depricated)
4728
PA-SANDIA-SECUREID 6
4729
PA-SESAME 7
4730
PA-OSF-DCE 8
4731
PA-CYBERSAFE-SECUREID 9
4732
PA-AFS3-SALT 10
4733
PA-ETYPE-INFO 11
4734
PA-SAM-CHALLENGE 12 (sam/otp)
4735
PA-SAM-RESPONSE 13 (sam/otp)
4736
PA-PK-AS-REQ 14 (pkinit)
4737
PA-PK-AS-REP 15 (pkinit)
4738
PA-USE-SPECIFIED-KVNO 20
4739
PA-SAM-REDIRECT 21 (sam/otp)
4740
PA-GET-FROM-TYPED-DATA 22
4741
PA-SAM-ETYPE-INFO 23 (sam/otp)
4742
4743
data-type value form of typed-data
4744
4745
<reserved> 1-21
4746
TD-PADATA 22
4747
TD-PKINIT-CMS-CERTIFICATES 101 CertificateSet from CMS
4748
TD-KRB-PRINCIPAL 102
4749
TD-KRB-REALM 103
4750
TD-TRUSTED-CERTIFIERS 104
4751
4752
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4753
4754
4755
4756
4757
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4758
4759
TD-CERTIFICATE-INDEX 105
4760
4761
authorization data type ad-type value
4762
AD-IF-RELEVANT 1
4763
AD-INTENDED-FOR-SERVER 2
4764
AD-INTENDED-FOR-APPLICATION-CLASS 3
4765
AD-KDC-ISSUED 4
4766
AD-OR 5
4767
AD-MANDATORY-TICKET-EXTENSIONS 6
4768
AD-IN-TICKET-EXTENSIONS 7
4769
reserved values 8-63
4770
OSF-DCE 64
4771
SESAME 65
4772
AD-OSF-DCE-PKI-CERTID 66 (hemsath@us.ibm.com)
4773
4774
Ticket Extension Types
4775
4776
TE-TYPE-NULL 0 Null ticket extension
4777
TE-TYPE-EXTERNAL-ADATA 1 Integrity protected authorization data
4778
<reserved> 2 TE-TYPE-PKCROSS-KDC (I have reservations)
4779
TE-TYPE-PKCROSS-CLIENT 3 PKCROSS cross realm key ticket
4780
TE-TYPE-CYBERSAFE-EXT 4 Assigned to CyberSafe Corp
4781
<reserved> 5 TE-TYPE-DEST-HOST (I have reservations)
4782
4783
alternate authentication type method-type value
4784
reserved values 0-63
4785
ATT-CHALLENGE-RESPONSE 64
4786
4787
transited encoding type tr-type value
4788
DOMAIN-X500-COMPRESS 1
4789
reserved values all others
4790
4791
Label Value Meaning or MIT code
4792
4793
pvno 5 current Kerberos protocol version number
4794
4795
message types
4796
4797
KRB_AS_REQ 10 Request for initial authentication
4798
KRB_AS_REP 11 Response to KRB_AS_REQ request
4799
KRB_TGS_REQ 12 Request for authentication based on TGT
4800
KRB_TGS_REP 13 Response to KRB_TGS_REQ request
4801
KRB_AP_REQ 14 application request to server
4802
KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL
4803
KRB_SAFE 20 Safe (checksummed) application message
4804
KRB_PRIV 21 Private (encrypted) application message
4805
KRB_CRED 22 Private (encrypted) message to forward credentials
4806
KRB_ERROR 30 Error response
4807
4808
name types
4809
4810
KRB_NT_UNKNOWN 0 Name type not known
4811
KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE, or for users
4812
KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt)
4813
KRB_NT_SRV_HST 3 Service with host name as instance (telnet, rcommands)
4814
KRB_NT_SRV_XHST 4 Service with host as remaining components
4815
KRB_NT_UID 5 Unique ID
4816
KRB_NT_X500_PRINCIPAL 6 Encoded X.509 Distingished name [RFC 2253]
4817
4818
4819
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4820
4821
4822
4823
4824
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4825
4826
error codes
4827
4828
KDC_ERR_NONE 0 No error
4829
KDC_ERR_NAME_EXP 1 Client's entry in database has expired
4830
KDC_ERR_SERVICE_EXP 2 Server's entry in database has expired
4831
KDC_ERR_BAD_PVNO 3 Requested prot vers number not supported
4832
KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in old master key
4833
KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in old master key
4834
KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in Kerberos database
4835
KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in Kerberos database
4836
KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries in database
4837
KDC_ERR_NULL_KEY 9 The client or server has a null key
4838
KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for postdating
4839
KDC_ERR_NEVER_VALID 11 Requested start time is later than end time
4840
KDC_ERR_POLICY 12 KDC policy rejects request
4841
KDC_ERR_BADOPTION 13 KDC cannot accommodate requested option
4842
KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for encryption type
4843
KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for checksum type
4844
KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for padata type
4845
KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for transited type
4846
KDC_ERR_CLIENT_REVOKED 18 Clients credentials have been revoked
4847
KDC_ERR_SERVICE_REVOKED 19 Credentials for server have been revoked
4848
KDC_ERR_TGT_REVOKED 20 TGT has been revoked
4849
KDC_ERR_CLIENT_NOTYET 21 Client not yet valid - try again later
4850
KDC_ERR_SERVICE_NOTYET 22 Server not yet valid - try again later
4851
KDC_ERR_KEY_EXPIRED 23 Password has expired - change password
4852
KDC_ERR_PREAUTH_FAILED 24 Pre-authentication information was invalid
4853
KDC_ERR_PREAUTH_REQUIRED 25 Additional pre-authenticationrequired [40]
4854
KDC_ERR_SERVER_NOMATCH 26 Requested server and ticket don't match
4855
KDC_ERR_MUST_USE_USER2USER 27 Server principal valid for user2user only
4856
KDC_ERR_PATH_NOT_ACCPETED 28 KDC Policy rejects transited path
4857
KDC_ERR_SVC_UNAVAILABLE 29 A service is not available
4858
KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on decrypted field failed
4859
KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired
4860
KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid
4861
KRB_AP_ERR_REPEAT 34 Request is a replay
4862
KRB_AP_ERR_NOT_US 35 The ticket isn't for us
4863
KRB_AP_ERR_BADMATCH 36 Ticket and authenticator don't match
4864
KRB_AP_ERR_SKEW 37 Clock skew too great
4865
KRB_AP_ERR_BADADDR 38 Incorrect net address
4866
KRB_AP_ERR_BADVERSION 39 Protocol version mismatch
4867
KRB_AP_ERR_MSG_TYPE 40 Invalid msg type
4868
KRB_AP_ERR_MODIFIED 41 Message stream modified
4869
KRB_AP_ERR_BADORDER 42 Message out of order
4870
KRB_AP_ERR_BADKEYVER 44 Specified version of key is not available
4871
KRB_AP_ERR_NOKEY 45 Service key not available
4872
KRB_AP_ERR_MUT_FAIL 46 Mutual authentication failed
4873
KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction
4874
KRB_AP_ERR_METHOD 48 Alternative authentication method required
4875
KRB_AP_ERR_BADSEQ 49 Incorrect sequence number in message
4876
KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of checksum in message
4877
KRB_AP_PATH_NOT_ACCEPTED 51 Policy rejects transited path
4878
KRB_ERR_RESPONSE_TOO_BIG 52 Response too big for UDP, retry with TCP
4879
KRB_ERR_GENERIC 60 Generic error (description in e-text)
4880
KRB_ERR_FIELD_TOOLONG 61 Field is too long for this implementation
4881
KDC_ERROR_CLIENT_NOT_TRUSTED 62 (pkinit)
4882
KDC_ERROR_KDC_NOT_TRUSTED 63 (pkinit)
4883
KDC_ERROR_INVALID_SIG 64 (pkinit)
4884
KDC_ERR_KEY_TOO_WEAK 65 (pkinit)
4885
4886
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4887
4888
4889
4890
4891
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4892
4893
KDC_ERR_CERTIFICATE_MISMATCH 66 (pkinit)
4894
KRB_AP_ERR_NO_TGT 67 (user-to-user)
4895
KDC_ERR_WRONG_REALM 68 (user-to-user)
4896
KRB_AP_ERR_USER_TO_USER_REQUIRED 69 (user-to-user)
4897
KDC_ERR_CANT_VERIFY_CERTIFICATE 70 (pkinit)
4898
KDC_ERR_INVALID_CERTIFICATE 71 (pkinit)
4899
KDC_ERR_REVOKED_CERTIFICATE 72 (pkinit)
4900
KDC_ERR_REVOCATION_STATUS_UNKNOWN 73 (pkinit)
4901
KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74 (pkinit)
4902
KDC_ERR_CLIENT_NAME_MISMATCH 75 (pkinit)
4903
KDC_ERR_KDC_NAME_MISMATCH 76 (pkinit)
4904
4905
9. Interoperability requirements
4906
4907
Version 5 of the Kerberos protocol supports a myriad of options. Among
4908
these are multiple encryption and checksum types, alternative encoding
4909
schemes for the transited field, optional mechanisms for
4910
pre-authentication, the handling of tickets with no addresses, options
4911
for mutual authentication, user to user authentication, support for
4912
proxies, forwarding, postdating, and renewing tickets, the format of
4913
realm names, and the handling of authorization data.
4914
4915
In order to ensure the interoperability of realms, it is necessary to
4916
define a minimal configuration which must be supported by all
4917
implementations. This minimal configuration is subject to change as
4918
technology does. For example, if at some later date it is discovered
4919
that one of the required encryption or checksum algorithms is not
4920
secure, it will be replaced.
4921
4922
9.1. Specification 2
4923
4924
This section defines the second specification of these options.
4925
Implementations which are configured in this way can be said to support
4926
Kerberos Version 5 Specification 2 (5.1). Specification 1 (depricated)
4927
may be found in RFC1510.
4928
4929
Transport
4930
4931
TCP/IP and UDP/IP transport must be supported by KDCs claiming
4932
conformance to specification 2. Kerberos clients claiming conformance
4933
to specification 2 must support UDP/IP transport for messages with the
4934
KDC and should support TCP/IP transport.
4935
4936
Encryption and checksum methods
4937
4938
The following encryption and checksum mechanisms must be supported.
4939
Implementations may support other mechanisms as well, but the
4940
additional mechanisms may only be used when communicating with
4941
principals known to also support them: This list is to be determined.
4942
4943
Encryption: DES-CBC-MD5, one triple des variant (tbd)
4944
Checksums: CRC-32, DES-MAC, DES-MAC-K, and DES-MD5 (tbd)
4945
4946
Realm Names
4947
4948
All implementations must understand hierarchical realms in both the
4949
Internet Domain and the X.500 style. When a ticket granting ticket for
4950
an unknown realm is requested, the KDC must be able to determine the
4951
names of the intermediate realms between the KDCs realm and the
4952
4953
Neuman, Ts'o, Kohl Expires: 10 September, 2000
4954
4955
4956
4957
4958
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
4959
4960
requested realm.
4961
4962
Transited field encoding
4963
4964
DOMAIN-X500-COMPRESS (described in section 3.3.3.2) must be supported.
4965
Alternative encodings may be supported, but they may be used only when
4966
that encoding is supported by ALL intermediate realms.
4967
4968
Pre-authentication methods
4969
4970
The TGS-REQ method must be supported. The TGS-REQ method is not used on
4971
the initial request. The PA-ENC-TIMESTAMP method must be supported by
4972
clients but whether it is enabled by default may be determined on a
4973
realm by realm basis. If not used in the initial request and the error
4974
KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENC-TIMESTAMP as an
4975
acceptable method, the client should retry the initial request using
4976
the PA-ENC-TIMESTAMP preauthentication method. Servers need not support
4977
the PA-ENC-TIMESTAMP method, but if not supported the server should
4978
ignore the presence of PA-ENC-TIMESTAMP pre-authentication in a
4979
request.
4980
4981
Mutual authentication
4982
4983
Mutual authentication (via the KRB_AP_REP message) must be supported.
4984
4985
Ticket addresses and flags
4986
4987
All KDC's must pass on tickets that carry no addresses (i.e. if a TGT
4988
contains no addresses, the KDC will return derivative tickets), but
4989
each realm may set its own policy for issuing such tickets, and each
4990
application server will set its own policy with respect to accepting
4991
them.
4992
4993
Proxies and forwarded tickets must be supported. Individual realms and
4994
application servers can set their own policy on when such tickets will
4995
be accepted.
4996
4997
All implementations must recognize renewable and postdated tickets, but
4998
need not actually implement them. If these options are not supported,
4999
the starttime and endtime in the ticket shall specify a ticket's entire
5000
useful life. When a postdated ticket is decoded by a server, all
5001
implementations shall make the presence of the postdated flag visible
5002
to the calling server.
5003
5004
User-to-user authentication
5005
5006
Support for user to user authentication (via the ENC-TKT-IN-SKEY KDC
5007
option) must be provided by implementations, but individual realms may
5008
decide as a matter of policy to reject such requests on a per-principal
5009
or realm-wide basis.
5010
5011
Authorization data
5012
5013
Implementations must pass all authorization data subfields from
5014
ticket-granting tickets to any derivative tickets unless directed to
5015
suppress a subfield as part of the definition of that registered
5016
subfield type (it is never incorrect to pass on a subfield, and no
5017
registered subfield types presently specify suppression at the KDC).
5018
5019
5020
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5021
5022
5023
5024
5025
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5026
5027
Implementations must make the contents of any authorization data
5028
subfields available to the server when a ticket is used.
5029
Implementations are not required to allow clients to specify the
5030
contents of the authorization data fields.
5031
5032
Constant ranges
5033
5034
All protocol constants are constrained to 32 bit (signed) values unless
5035
further constrained by the protocol definition. This limit is provided
5036
to allow implementations to make assumptions about the maximum values
5037
that will be received for these constants. Implementation receiving
5038
values outside this range may reject the request, but they must recover
5039
cleanly.
5040
5041
9.2. Recommended KDC values
5042
5043
Following is a list of recommended values for a KDC implementation,
5044
based on the list of suggested configuration constants (see section
5045
4.4).
5046
5047
minimum lifetime 5 minutes
5048
maximum renewable lifetime 1 week
5049
maximum ticket lifetime 1 day
5050
empty addresses only when suitable restrictions appear
5051
in authorization data
5052
proxiable, etc. Allowed.
5053
5054
10. REFERENCES
5055
5056
[NT94] B. Clifford Neuman and Theodore Y. Ts'o, "An Authenti-
5057
cation Service for Computer Networks," IEEE Communica-
5058
tions Magazine, Vol. 32(9), pp. 33-38 (September 1994).
5059
5060
[MNSS87] S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H.
5061
Saltzer, Section E.2.1: Kerberos Authentication and
5062
Authorization System, M.I.T. Project Athena, Cambridge,
5063
Massachusetts (December 21, 1987).
5064
5065
[SNS88] J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Ker-
5066
beros: An Authentication Service for Open Network Sys-
5067
tems," pp. 191-202 in Usenix Conference Proceedings,
5068
Dallas, Texas (February, 1988).
5069
5070
[NS78] Roger M. Needham and Michael D. Schroeder, "Using
5071
Encryption for Authentication in Large Networks of Com-
5072
puters," Communications of the ACM, Vol. 21(12),
5073
pp. 993-999 (December, 1978).
5074
5075
[DS81] Dorothy E. Denning and Giovanni Maria Sacco, "Time-
5076
stamps in Key Distribution Protocols," Communications
5077
of the ACM, Vol. 24(8), pp. 533-536 (August 1981).
5078
5079
[KNT92] John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o,
5080
"The Evolution of the Kerberos Authentication Service,"
5081
in an IEEE Computer Society Text soon to be published
5082
(June 1992).
5083
5084
[Neu93] B. Clifford Neuman, "Proxy-Based Authorization and
5085
Accounting for Distributed Systems," in Proceedings of
5086
5087
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5088
5089
5090
5091
5092
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5093
5094
the 13th International Conference on Distributed Com-
5095
puting Systems, Pittsburgh, PA (May, 1993).
5096
5097
[DS90] Don Davis and Ralph Swick, "Workstation Services and
5098
Kerberos Authentication at Project Athena," Technical
5099
Memorandum TM-424, MIT Laboratory for Computer Science
5100
(February 1990).
5101
5102
[LGDSR87] P. J. Levine, M. R. Gretzinger, J. M. Diaz, W. E. Som-
5103
merfeld, and K. Raeburn, Section E.1: Service Manage-
5104
ment System, M.I.T. Project Athena, Cambridge, Mas-
5105
sachusetts (1987).
5106
5107
[X509-88] CCITT, Recommendation X.509: The Directory Authentica-
5108
tion Framework, December 1988.
5109
5110
[Pat92]. J. Pato, Using Pre-Authentication to Avoid Password
5111
Guessing Attacks, Open Software Foundation DCE Request
5112
for Comments 26 (December 1992).
5113
5114
[DES77] National Bureau of Standards, U.S. Department of Com-
5115
merce, "Data Encryption Standard," Federal Information
5116
Processing Standards Publication 46, Washington, DC
5117
(1977).
5118
5119
[DESM80] National Bureau of Standards, U.S. Department of Com-
5120
merce, "DES Modes of Operation," Federal Information
5121
Processing Standards Publication 81, Springfield, VA
5122
(December 1980).
5123
5124
[SG92] Stuart G. Stubblebine and Virgil D. Gligor, "On Message
5125
Integrity in Cryptographic Protocols," in Proceedings
5126
of the IEEE Symposium on Research in Security and
5127
Privacy, Oakland, California (May 1992).
5128
5129
[IS3309] International Organization for Standardization, "ISO
5130
Information Processing Systems - Data Communication -
5131
High-Level Data Link Control Procedure - Frame Struc-
5132
ture," IS 3309 (October 1984). 3rd Edition.
5133
5134
[MD4-92] R. Rivest, "The MD4 Message Digest Algorithm," RFC
5135
1320, MIT Laboratory for Computer Science (April
5136
1992).
5137
5138
[MD5-92] R. Rivest, "The MD5 Message Digest Algorithm," RFC
5139
1321, MIT Laboratory for Computer Science (April
5140
1992).
5141
5142
[KBC96] H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: Keyed-
5143
Hashing for Message Authentication," Working Draft
5144
draft-ietf-ipsec-hmac-md5-01.txt, (August 1996).
5145
5146
[Horowitz96] Horowitz, M., "Key Derivation for Authentication,
5147
Integrity, and Privacy", draft-horowitz-key-derivation-02.txt,
5148
August 1998.
5149
5150
[HorowitzB96] Horowitz, M., "Key Derivation for Kerberos V5", draft-
5151
horowitz-kerb-key-derivation-01.txt, September 1998.
5152
5153
5154
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5155
5156
5157
5158
5159
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5160
5161
[Krawczyk96] Krawczyk, H., Bellare, and M., Canetti, R., "HMAC:
5162
Keyed-Hashing for Message Authentication", draft-ietf-ipsec-hmac-
5163
md5-01.txt, August, 1996.
5164
5165
A. Pseudo-code for protocol processing
5166
5167
This appendix provides pseudo-code describing how the messages are to
5168
be constructed and interpreted by clients and servers.
5169
5170
A.1. KRB_AS_REQ generation
5171
5172
request.pvno := protocol version; /* pvno = 5 */
5173
request.msg-type := message type; /* type = KRB_AS_REQ */
5174
5175
if(pa_enc_timestamp_required) then
5176
request.padata.padata-type = PA-ENC-TIMESTAMP;
5177
get system_time;
5178
padata-body.patimestamp,pausec = system_time;
5179
encrypt padata-body into request.padata.padata-value
5180
using client.key; /* derived from password */
5181
endif
5182
5183
body.kdc-options := users's preferences;
5184
body.cname := user's name;
5185
body.realm := user's realm;
5186
body.sname := service's name; /* usually "krbtgt",
5187
"localrealm" */
5188
5189
if (body.kdc-options.POSTDATED is set) then
5190
body.from := requested starting time;
5191
else
5192
omit body.from;
5193
endif
5194
body.till := requested end time;
5195
if (body.kdc-options.RENEWABLE is set) then
5196
body.rtime := requested final renewal time;
5197
endif
5198
body.nonce := random_nonce();
5199
body.etype := requested etypes;
5200
if (user supplied addresses) then
5201
body.addresses := user's addresses;
5202
else
5203
omit body.addresses;
5204
endif
5205
omit body.enc-authorization-data;
5206
request.req-body := body;
5207
5208
kerberos := lookup(name of local kerberos server (or servers));
5209
send(packet,kerberos);
5210
5211
wait(for response);
5212
if (timed_out) then
5213
retry or use alternate server;
5214
endif
5215
5216
A.2. KRB_AS_REQ verification and KRB_AS_REP generation
5217
5218
decode message into req;
5219
5220
5221
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5222
5223
5224
5225
5226
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5227
5228
client := lookup(req.cname,req.realm);
5229
server := lookup(req.sname,req.realm);
5230
5231
get system_time;
5232
kdc_time := system_time.seconds;
5233
5234
if (!client) then
5235
/* no client in Database */
5236
error_out(KDC_ERR_C_PRINCIPAL_UNKNOWN);
5237
endif
5238
if (!server) then
5239
/* no server in Database */
5240
error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
5241
endif
5242
5243
if(client.pa_enc_timestamp_required and
5244
pa_enc_timestamp not present) then
5245
error_out(KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP));
5246
endif
5247
5248
if(pa_enc_timestamp present) then
5249
decrypt req.padata-value into decrypted_enc_timestamp
5250
using client.key;
5251
using auth_hdr.authenticator.subkey;
5252
if (decrypt_error()) then
5253
error_out(KRB_AP_ERR_BAD_INTEGRITY);
5254
if(decrypted_enc_timestamp is not within allowable skew)
5255
then
5256
error_out(KDC_ERR_PREAUTH_FAILED);
5257
endif
5258
if(decrypted_enc_timestamp and usec is replay)
5259
error_out(KDC_ERR_PREAUTH_FAILED);
5260
endif
5261
add decrypted_enc_timestamp and usec to replay cache;
5262
endif
5263
5264
use_etype := first supported etype in req.etypes;
5265
5266
if (no support for req.etypes) then
5267
error_out(KDC_ERR_ETYPE_NOSUPP);
5268
endif
5269
5270
new_tkt.vno := ticket version; /* = 5 */
5271
new_tkt.sname := req.sname;
5272
new_tkt.srealm := req.srealm;
5273
reset all flags in new_tkt.flags;
5274
5275
/* It should be noted that local policy may affect the */
5276
/* processing of any of these flags. For example, some */
5277
/* realms may refuse to issue renewable tickets */
5278
5279
if (req.kdc-options.FORWARDABLE is set) then
5280
set new_tkt.flags.FORWARDABLE;
5281
endif
5282
if (req.kdc-options.PROXIABLE is set) then
5283
set new_tkt.flags.PROXIABLE;
5284
endif
5285
5286
if (req.kdc-options.ALLOW-POSTDATE is set) then
5287
5288
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5289
5290
5291
5292
5293
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5294
5295
set new_tkt.flags.MAY-POSTDATE;
5296
endif
5297
if ((req.kdc-options.RENEW is set) or
5298
(req.kdc-options.VALIDATE is set) or
5299
(req.kdc-options.PROXY is set) or
5300
(req.kdc-options.FORWARDED is set) or
5301
(req.kdc-options.ENC-TKT-IN-SKEY is set)) then
5302
error_out(KDC_ERR_BADOPTION);
5303
endif
5304
5305
new_tkt.session := random_session_key();
5306
new_tkt.cname := req.cname;
5307
new_tkt.crealm := req.crealm;
5308
new_tkt.transited := empty_transited_field();
5309
5310
new_tkt.authtime := kdc_time;
5311
5312
if (req.kdc-options.POSTDATED is set) then
5313
if (against_postdate_policy(req.from)) then
5314
error_out(KDC_ERR_POLICY);
5315
endif
5316
set new_tkt.flags.POSTDATED;
5317
set new_tkt.flags.INVALID;
5318
new_tkt.starttime := req.from;
5319
else
5320
omit new_tkt.starttime; /* treated as authtime when omitted */
5321
endif
5322
if (req.till = 0) then
5323
till := infinity;
5324
else
5325
till := req.till;
5326
endif
5327
5328
new_tkt.endtime := min(till,
5329
new_tkt.starttime+client.max_life,
5330
new_tkt.starttime+server.max_life,
5331
new_tkt.starttime+max_life_for_realm);
5332
5333
if ((req.kdc-options.RENEWABLE-OK is set) and
5334
(new_tkt.endtime < req.till)) then
5335
/* we set the RENEWABLE option for later processing */
5336
set req.kdc-options.RENEWABLE;
5337
req.rtime := req.till;
5338
endif
5339
5340
if (req.rtime = 0) then
5341
rtime := infinity;
5342
else
5343
rtime := req.rtime;
5344
endif
5345
5346
if (req.kdc-options.RENEWABLE is set) then
5347
set new_tkt.flags.RENEWABLE;
5348
new_tkt.renew-till := min(rtime,
5349
new_tkt.starttime+client.max_rlife,
5350
new_tkt.starttime+server.max_rlife,
5351
new_tkt.starttime+max_rlife_for_realm);
5352
else
5353
omit new_tkt.renew-till; /* only present if RENEWABLE */
5354
5355
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5356
5357
5358
5359
5360
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5361
5362
endif
5363
5364
if (req.addresses) then
5365
new_tkt.caddr := req.addresses;
5366
else
5367
omit new_tkt.caddr;
5368
endif
5369
5370
new_tkt.authorization_data := empty_authorization_data();
5371
5372
encode to-be-encrypted part of ticket into OCTET STRING;
5373
new_tkt.enc-part := encrypt OCTET STRING
5374
using etype_for_key(server.key), server.key, server.p_kvno;
5375
5376
/* Start processing the response */
5377
5378
resp.pvno := 5;
5379
resp.msg-type := KRB_AS_REP;
5380
resp.cname := req.cname;
5381
resp.crealm := req.realm;
5382
resp.ticket := new_tkt;
5383
5384
resp.key := new_tkt.session;
5385
resp.last-req := fetch_last_request_info(client);
5386
resp.nonce := req.nonce;
5387
resp.key-expiration := client.expiration;
5388
resp.flags := new_tkt.flags;
5389
5390
resp.authtime := new_tkt.authtime;
5391
resp.starttime := new_tkt.starttime;
5392
resp.endtime := new_tkt.endtime;
5393
5394
if (new_tkt.flags.RENEWABLE) then
5395
resp.renew-till := new_tkt.renew-till;
5396
endif
5397
5398
resp.realm := new_tkt.realm;
5399
resp.sname := new_tkt.sname;
5400
5401
resp.caddr := new_tkt.caddr;
5402
5403
encode body of reply into OCTET STRING;
5404
5405
resp.enc-part := encrypt OCTET STRING
5406
using use_etype, client.key, client.p_kvno;
5407
send(resp);
5408
5409
A.3. KRB_AS_REP verification
5410
5411
decode response into resp;
5412
5413
if (resp.msg-type = KRB_ERROR) then
5414
if(error = KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP)) then
5415
set pa_enc_timestamp_required;
5416
goto KRB_AS_REQ;
5417
endif
5418
process_error(resp);
5419
return;
5420
endif
5421
5422
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5423
5424
5425
5426
5427
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5428
5429
5430
/* On error, discard the response, and zero the session key */
5431
/* from the response immediately */
5432
5433
key = get_decryption_key(resp.enc-part.kvno, resp.enc-part.etype,
5434
resp.padata);
5435
unencrypted part of resp := decode of decrypt of resp.enc-part
5436
using resp.enc-part.etype and key;
5437
zero(key);
5438
5439
if (common_as_rep_tgs_rep_checks fail) then
5440
destroy resp.key;
5441
return error;
5442
endif
5443
5444
if near(resp.princ_exp) then
5445
print(warning message);
5446
endif
5447
save_for_later(ticket,session,client,server,times,flags);
5448
5449
A.4. KRB_AS_REP and KRB_TGS_REP common checks
5450
5451
if (decryption_error() or
5452
(req.cname != resp.cname) or
5453
(req.realm != resp.crealm) or
5454
(req.sname != resp.sname) or
5455
(req.realm != resp.realm) or
5456
(req.nonce != resp.nonce) or
5457
(req.addresses != resp.caddr)) then
5458
destroy resp.key;
5459
return KRB_AP_ERR_MODIFIED;
5460
endif
5461
5462
/* make sure no flags are set that shouldn't be, and that all that */
5463
/* should be are set */
5464
if (!check_flags_for_compatability(req.kdc-options,resp.flags)) then
5465
destroy resp.key;
5466
return KRB_AP_ERR_MODIFIED;
5467
endif
5468
5469
if ((req.from = 0) and
5470
(resp.starttime is not within allowable skew)) then
5471
destroy resp.key;
5472
return KRB_AP_ERR_SKEW;
5473
endif
5474
if ((req.from != 0) and (req.from != resp.starttime)) then
5475
destroy resp.key;
5476
return KRB_AP_ERR_MODIFIED;
5477
endif
5478
if ((req.till != 0) and (resp.endtime > req.till)) then
5479
destroy resp.key;
5480
return KRB_AP_ERR_MODIFIED;
5481
endif
5482
5483
if ((req.kdc-options.RENEWABLE is set) and
5484
(req.rtime != 0) and (resp.renew-till > req.rtime)) then
5485
destroy resp.key;
5486
return KRB_AP_ERR_MODIFIED;
5487
endif
5488
5489
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5490
5491
5492
5493
5494
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5495
5496
if ((req.kdc-options.RENEWABLE-OK is set) and
5497
(resp.flags.RENEWABLE) and
5498
(req.till != 0) and
5499
(resp.renew-till > req.till)) then
5500
destroy resp.key;
5501
return KRB_AP_ERR_MODIFIED;
5502
endif
5503
5504
A.5. KRB_TGS_REQ generation
5505
5506
/* Note that make_application_request might have to recursivly */
5507
/* call this routine to get the appropriate ticket-granting ticket */
5508
5509
request.pvno := protocol version; /* pvno = 5 */
5510
request.msg-type := message type; /* type = KRB_TGS_REQ */
5511
5512
body.kdc-options := users's preferences;
5513
/* If the TGT is not for the realm of the end-server */
5514
/* then the sname will be for a TGT for the end-realm */
5515
/* and the realm of the requested ticket (body.realm) */
5516
/* will be that of the TGS to which the TGT we are */
5517
/* sending applies */
5518
body.sname := service's name;
5519
body.realm := service's realm;
5520
5521
if (body.kdc-options.POSTDATED is set) then
5522
body.from := requested starting time;
5523
else
5524
omit body.from;
5525
endif
5526
body.till := requested end time;
5527
if (body.kdc-options.RENEWABLE is set) then
5528
body.rtime := requested final renewal time;
5529
endif
5530
body.nonce := random_nonce();
5531
body.etype := requested etypes;
5532
if (user supplied addresses) then
5533
body.addresses := user's addresses;
5534
else
5535
omit body.addresses;
5536
endif
5537
5538
body.enc-authorization-data := user-supplied data;
5539
if (body.kdc-options.ENC-TKT-IN-SKEY) then
5540
body.additional-tickets_ticket := second TGT;
5541
endif
5542
5543
request.req-body := body;
5544
check := generate_checksum (req.body,checksumtype);
5545
5546
request.padata[0].padata-type := PA-TGS-REQ;
5547
request.padata[0].padata-value := create a KRB_AP_REQ using
5548
the TGT and checksum
5549
5550
/* add in any other padata as required/supplied */
5551
5552
kerberos := lookup(name of local kerberose server (or servers));
5553
send(packet,kerberos);
5554
5555
5556
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5557
5558
5559
5560
5561
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5562
5563
wait(for response);
5564
if (timed_out) then
5565
retry or use alternate server;
5566
endif
5567
5568
A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation
5569
5570
/* note that reading the application request requires first
5571
determining the server for which a ticket was issued, and
5572
choosing the correct key for decryption. The name of the
5573
server appears in the plaintext part of the ticket. */
5574
5575
if (no KRB_AP_REQ in req.padata) then
5576
error_out(KDC_ERR_PADATA_TYPE_NOSUPP);
5577
endif
5578
verify KRB_AP_REQ in req.padata;
5579
5580
/* Note that the realm in which the Kerberos server is
5581
operating is determined by the instance from the
5582
ticket-granting ticket. The realm in the ticket-granting
5583
ticket is the realm under which the ticket granting
5584
ticket was issued. It is possible for a single Kerberos
5585
server to support more than one realm. */
5586
5587
auth_hdr := KRB_AP_REQ;
5588
tgt := auth_hdr.ticket;
5589
5590
if (tgt.sname is not a TGT for local realm and is not req.sname)
5591
then
5592
error_out(KRB_AP_ERR_NOT_US);
5593
5594
realm := realm_tgt_is_for(tgt);
5595
5596
decode remainder of request;
5597
5598
if (auth_hdr.authenticator.cksum is missing) then
5599
error_out(KRB_AP_ERR_INAPP_CKSUM);
5600
endif
5601
5602
if (auth_hdr.authenticator.cksum type is not supported) then
5603
error_out(KDC_ERR_SUMTYPE_NOSUPP);
5604
endif
5605
if (auth_hdr.authenticator.cksum is not both collision-proof
5606
and keyed) then
5607
error_out(KRB_AP_ERR_INAPP_CKSUM);
5608
endif
5609
5610
set computed_checksum := checksum(req);
5611
if (computed_checksum != auth_hdr.authenticatory.cksum) then
5612
error_out(KRB_AP_ERR_MODIFIED);
5613
endif
5614
5615
server := lookup(req.sname,realm);
5616
5617
if (!server) then
5618
if (is_foreign_tgt_name(req.sname)) then
5619
server := best_intermediate_tgs(req.sname);
5620
else
5621
/* no server in Database */
5622
5623
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5624
5625
5626
5627
5628
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5629
5630
error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
5631
endif
5632
endif
5633
5634
session := generate_random_session_key();
5635
5636
use_etype := first supported etype in req.etypes;
5637
5638
if (no support for req.etypes) then
5639
error_out(KDC_ERR_ETYPE_NOSUPP);
5640
endif
5641
5642
new_tkt.vno := ticket version; /* = 5 */
5643
new_tkt.sname := req.sname;
5644
new_tkt.srealm := realm;
5645
reset all flags in new_tkt.flags;
5646
5647
/* It should be noted that local policy may affect the */
5648
/* processing of any of these flags. For example, some */
5649
/* realms may refuse to issue renewable tickets */
5650
5651
new_tkt.caddr := tgt.caddr;
5652
resp.caddr := NULL; /* We only include this if they change */
5653
if (req.kdc-options.FORWARDABLE is set) then
5654
if (tgt.flags.FORWARDABLE is reset) then
5655
error_out(KDC_ERR_BADOPTION);
5656
endif
5657
set new_tkt.flags.FORWARDABLE;
5658
endif
5659
if (req.kdc-options.FORWARDED is set) then
5660
if (tgt.flags.FORWARDABLE is reset) then
5661
error_out(KDC_ERR_BADOPTION);
5662
endif
5663
set new_tkt.flags.FORWARDED;
5664
new_tkt.caddr := req.addresses;
5665
resp.caddr := req.addresses;
5666
endif
5667
if (tgt.flags.FORWARDED is set) then
5668
set new_tkt.flags.FORWARDED;
5669
endif
5670
5671
if (req.kdc-options.PROXIABLE is set) then
5672
if (tgt.flags.PROXIABLE is reset)
5673
error_out(KDC_ERR_BADOPTION);
5674
endif
5675
set new_tkt.flags.PROXIABLE;
5676
endif
5677
if (req.kdc-options.PROXY is set) then
5678
if (tgt.flags.PROXIABLE is reset) then
5679
error_out(KDC_ERR_BADOPTION);
5680
endif
5681
set new_tkt.flags.PROXY;
5682
new_tkt.caddr := req.addresses;
5683
resp.caddr := req.addresses;
5684
endif
5685
5686
if (req.kdc-options.ALLOW-POSTDATE is set) then
5687
if (tgt.flags.MAY-POSTDATE is reset)
5688
error_out(KDC_ERR_BADOPTION);
5689
5690
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5691
5692
5693
5694
5695
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5696
5697
endif
5698
set new_tkt.flags.MAY-POSTDATE;
5699
endif
5700
if (req.kdc-options.POSTDATED is set) then
5701
if (tgt.flags.MAY-POSTDATE is reset) then
5702
error_out(KDC_ERR_BADOPTION);
5703
endif
5704
set new_tkt.flags.POSTDATED;
5705
set new_tkt.flags.INVALID;
5706
if (against_postdate_policy(req.from)) then
5707
error_out(KDC_ERR_POLICY);
5708
endif
5709
new_tkt.starttime := req.from;
5710
endif
5711
5712
if (req.kdc-options.VALIDATE is set) then
5713
if (tgt.flags.INVALID is reset) then
5714
error_out(KDC_ERR_POLICY);
5715
endif
5716
if (tgt.starttime > kdc_time) then
5717
error_out(KRB_AP_ERR_NYV);
5718
endif
5719
if (check_hot_list(tgt)) then
5720
error_out(KRB_AP_ERR_REPEAT);
5721
endif
5722
tkt := tgt;
5723
reset new_tkt.flags.INVALID;
5724
endif
5725
5726
if (req.kdc-options.(any flag except ENC-TKT-IN-SKEY, RENEW,
5727
and those already processed) is set) then
5728
error_out(KDC_ERR_BADOPTION);
5729
endif
5730
5731
new_tkt.authtime := tgt.authtime;
5732
5733
if (req.kdc-options.RENEW is set) then
5734
/* Note that if the endtime has already passed, the ticket would */
5735
/* have been rejected in the initial authentication stage, so */
5736
/* there is no need to check again here */
5737
if (tgt.flags.RENEWABLE is reset) then
5738
error_out(KDC_ERR_BADOPTION);
5739
endif
5740
if (tgt.renew-till < kdc_time) then
5741
error_out(KRB_AP_ERR_TKT_EXPIRED);
5742
endif
5743
tkt := tgt;
5744
new_tkt.starttime := kdc_time;
5745
old_life := tgt.endttime - tgt.starttime;
5746
new_tkt.endtime := min(tgt.renew-till,
5747
new_tkt.starttime + old_life);
5748
else
5749
new_tkt.starttime := kdc_time;
5750
if (req.till = 0) then
5751
till := infinity;
5752
else
5753
till := req.till;
5754
endif
5755
new_tkt.endtime := min(till,
5756
5757
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5758
5759
5760
5761
5762
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5763
5764
new_tkt.starttime+client.max_life,
5765
new_tkt.starttime+server.max_life,
5766
new_tkt.starttime+max_life_for_realm,
5767
tgt.endtime);
5768
5769
if ((req.kdc-options.RENEWABLE-OK is set) and
5770
(new_tkt.endtime < req.till) and
5771
(tgt.flags.RENEWABLE is set) then
5772
/* we set the RENEWABLE option for later processing */
5773
set req.kdc-options.RENEWABLE;
5774
req.rtime := min(req.till, tgt.renew-till);
5775
endif
5776
endif
5777
5778
if (req.rtime = 0) then
5779
rtime := infinity;
5780
else
5781
rtime := req.rtime;
5782
endif
5783
5784
if ((req.kdc-options.RENEWABLE is set) and
5785
(tgt.flags.RENEWABLE is set)) then
5786
set new_tkt.flags.RENEWABLE;
5787
new_tkt.renew-till := min(rtime,
5788
new_tkt.starttime+client.max_rlife,
5789
new_tkt.starttime+server.max_rlife,
5790
new_tkt.starttime+max_rlife_for_realm,
5791
tgt.renew-till);
5792
else
5793
new_tkt.renew-till := OMIT; /* leave the
5794
renew-till field out */
5795
endif
5796
if (req.enc-authorization-data is present) then
5797
decrypt req.enc-authorization-data into
5798
decrypted_authorization_data
5799
using auth_hdr.authenticator.subkey;
5800
if (decrypt_error()) then
5801
error_out(KRB_AP_ERR_BAD_INTEGRITY);
5802
endif
5803
endif
5804
new_tkt.authorization_data :=
5805
req.auth_hdr.ticket.authorization_data +
5806
decrypted_authorization_data;
5807
5808
new_tkt.key := session;
5809
new_tkt.crealm := tgt.crealm;
5810
new_tkt.cname := req.auth_hdr.ticket.cname;
5811
5812
if (realm_tgt_is_for(tgt) := tgt.realm) then
5813
/* tgt issued by local realm */
5814
new_tkt.transited := tgt.transited;
5815
else
5816
/* was issued for this realm by some other realm */
5817
if (tgt.transited.tr-type not supported) then
5818
error_out(KDC_ERR_TRTYPE_NOSUPP);
5819
endif
5820
new_tkt.transited :=
5821
compress_transited(tgt.transited + tgt.realm)
5822
/* Don't check tranited field if TGT for foreign realm,
5823
5824
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5825
5826
5827
5828
5829
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5830
5831
* or requested not to check */
5832
if (is_not_foreign_tgt_name(new_tkt.server)
5833
&& req.kdc-options.DISABLE-TRANSITED-CHECK not
5834
set) then
5835
/* Check it, so end-server does not have to
5836
* but don't fail, end-server may still accept it */
5837
if (check_transited_field(new_tkt.transited) == OK)
5838
set new_tkt.flags.TRANSITED-POLICY-CHECKED;
5839
endif
5840
endif
5841
endif
5842
5843
encode encrypted part of new_tkt into OCTET STRING;
5844
if (req.kdc-options.ENC-TKT-IN-SKEY is set) then
5845
if (server not specified) then
5846
server = req.second_ticket.client;
5847
endif
5848
if ((req.second_ticket is not a TGT) or
5849
(req.second_ticket.client != server)) then
5850
error_out(KDC_ERR_POLICY);
5851
endif
5852
5853
new_tkt.enc-part := encrypt OCTET STRING using
5854
using etype_for_key(second-ticket.key),
5855
second-ticket.key;
5856
else
5857
new_tkt.enc-part := encrypt OCTET STRING
5858
using etype_for_key(server.key),
5859
server.key, server.p_kvno;
5860
endif
5861
5862
resp.pvno := 5;
5863
resp.msg-type := KRB_TGS_REP;
5864
resp.crealm := tgt.crealm;
5865
resp.cname := tgt.cname;
5866
resp.ticket := new_tkt;
5867
5868
resp.key := session;
5869
resp.nonce := req.nonce;
5870
resp.last-req := fetch_last_request_info(client);
5871
resp.flags := new_tkt.flags;
5872
5873
resp.authtime := new_tkt.authtime;
5874
resp.starttime := new_tkt.starttime;
5875
resp.endtime := new_tkt.endtime;
5876
5877
omit resp.key-expiration;
5878
5879
resp.sname := new_tkt.sname;
5880
resp.realm := new_tkt.realm;
5881
5882
if (new_tkt.flags.RENEWABLE) then
5883
resp.renew-till := new_tkt.renew-till;
5884
endif
5885
5886
encode body of reply into OCTET STRING;
5887
5888
if (req.padata.authenticator.subkey)
5889
resp.enc-part := encrypt OCTET STRING using use_etype,
5890
5891
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5892
5893
5894
5895
5896
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5897
5898
req.padata.authenticator.subkey;
5899
else resp.enc-part := encrypt OCTET STRING using
5900
use_etype, tgt.key;
5901
5902
send(resp);
5903
5904
A.7. KRB_TGS_REP verification
5905
5906
decode response into resp;
5907
5908
if (resp.msg-type = KRB_ERROR) then
5909
process_error(resp);
5910
return;
5911
endif
5912
5913
/* On error, discard the response, and zero the session key from
5914
the response immediately */
5915
5916
if (req.padata.authenticator.subkey)
5917
unencrypted part of resp := decode of decrypt of
5918
resp.enc-part
5919
using resp.enc-part.etype and subkey;
5920
else unencrypted part of resp := decode of decrypt of
5921
resp.enc-part
5922
using resp.enc-part.etype and
5923
tgt's session key;
5924
if (common_as_rep_tgs_rep_checks fail) then
5925
destroy resp.key;
5926
return error;
5927
endif
5928
5929
check authorization_data as necessary;
5930
save_for_later(ticket,session,client,server,times,flags);
5931
5932
A.8. Authenticator generation
5933
5934
body.authenticator-vno := authenticator vno; /* = 5 */
5935
body.cname, body.crealm := client name;
5936
if (supplying checksum) then
5937
body.cksum := checksum;
5938
endif
5939
get system_time;
5940
body.ctime, body.cusec := system_time;
5941
if (selecting sub-session key) then
5942
select sub-session key;
5943
body.subkey := sub-session key;
5944
endif
5945
if (using sequence numbers) then
5946
select initial sequence number;
5947
body.seq-number := initial sequence;
5948
endif
5949
5950
A.9. KRB_AP_REQ generation
5951
5952
obtain ticket and session_key from cache;
5953
5954
packet.pvno := protocol version; /* 5 */
5955
packet.msg-type := message type; /* KRB_AP_REQ */
5956
5957
5958
Neuman, Ts'o, Kohl Expires: 10 September, 2000
5959
5960
5961
5962
5963
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
5964
5965
if (desired(MUTUAL_AUTHENTICATION)) then
5966
set packet.ap-options.MUTUAL-REQUIRED;
5967
else
5968
reset packet.ap-options.MUTUAL-REQUIRED;
5969
endif
5970
if (using session key for ticket) then
5971
set packet.ap-options.USE-SESSION-KEY;
5972
else
5973
reset packet.ap-options.USE-SESSION-KEY;
5974
endif
5975
packet.ticket := ticket; /* ticket */
5976
generate authenticator;
5977
encode authenticator into OCTET STRING;
5978
encrypt OCTET STRING into packet.authenticator using session_key;
5979
5980
A.10. KRB_AP_REQ verification
5981
5982
receive packet;
5983
if (packet.pvno != 5) then
5984
either process using other protocol spec
5985
or error_out(KRB_AP_ERR_BADVERSION);
5986
endif
5987
if (packet.msg-type != KRB_AP_REQ) then
5988
error_out(KRB_AP_ERR_MSG_TYPE);
5989
endif
5990
if (packet.ticket.tkt_vno != 5) then
5991
either process using other protocol spec
5992
or error_out(KRB_AP_ERR_BADVERSION);
5993
endif
5994
if (packet.ap_options.USE-SESSION-KEY is set) then
5995
retrieve session key from ticket-granting ticket for
5996
packet.ticket.{sname,srealm,enc-part.etype};
5997
else
5998
retrieve service key for
5999
packet.ticket.{sname,srealm,enc-part.etype,enc-part.skvno};
6000
endif
6001
if (no_key_available) then
6002
if (cannot_find_specified_skvno) then
6003
error_out(KRB_AP_ERR_BADKEYVER);
6004
else
6005
error_out(KRB_AP_ERR_NOKEY);
6006
endif
6007
endif
6008
decrypt packet.ticket.enc-part into decr_ticket using
6009
retrieved key;
6010
if (decryption_error()) then
6011
error_out(KRB_AP_ERR_BAD_INTEGRITY);
6012
endif
6013
decrypt packet.authenticator into decr_authenticator
6014
using decr_ticket.key;
6015
if (decryption_error()) then
6016
error_out(KRB_AP_ERR_BAD_INTEGRITY);
6017
endif
6018
if (decr_authenticator.{cname,crealm} !=
6019
decr_ticket.{cname,crealm}) then
6020
error_out(KRB_AP_ERR_BADMATCH);
6021
endif
6022
if (decr_ticket.caddr is present) then
6023
if (sender_address(packet) is not in
6024
6025
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6026
6027
6028
6029
6030
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6031
6032
decr_ticket.caddr) then
6033
error_out(KRB_AP_ERR_BADADDR);
6034
endif
6035
elseif (application requires addresses) then
6036
error_out(KRB_AP_ERR_BADADDR);
6037
endif
6038
if (not in_clock_skew(decr_authenticator.ctime,
6039
decr_authenticator.cusec)) then
6040
error_out(KRB_AP_ERR_SKEW);
6041
endif
6042
if (repeated(decr_authenticator.{ctime,cusec,cname,crealm})) then
6043
error_out(KRB_AP_ERR_REPEAT);
6044
endif
6045
save_identifier(decr_authenticator.{ctime,cusec,cname,crealm});
6046
get system_time;
6047
if ((decr_ticket.starttime-system_time > CLOCK_SKEW) or
6048
(decr_ticket.flags.INVALID is set)) then
6049
/* it hasn't yet become valid */
6050
error_out(KRB_AP_ERR_TKT_NYV);
6051
endif
6052
if (system_time-decr_ticket.endtime > CLOCK_SKEW) then
6053
error_out(KRB_AP_ERR_TKT_EXPIRED);
6054
endif
6055
if (decr_ticket.transited) then
6056
/* caller may ignore the TRANSITED-POLICY-CHECKED and do
6057
* check anyway */
6058
if (decr_ticket.flags.TRANSITED-POLICY-CHECKED not set) then
6059
if (check_transited_field(decr_ticket.transited) then
6060
error_out(KDC_AP_PATH_NOT_ACCPETED);
6061
endif
6062
endif
6063
endif
6064
/* caller must check decr_ticket.flags for any pertinent details */
6065
return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED);
6066
6067
A.11. KRB_AP_REP generation
6068
6069
packet.pvno := protocol version; /* 5 */
6070
packet.msg-type := message type; /* KRB_AP_REP */
6071
6072
body.ctime := packet.ctime;
6073
body.cusec := packet.cusec;
6074
if (selecting sub-session key) then
6075
select sub-session key;
6076
body.subkey := sub-session key;
6077
endif
6078
if (using sequence numbers) then
6079
select initial sequence number;
6080
body.seq-number := initial sequence;
6081
endif
6082
6083
encode body into OCTET STRING;
6084
6085
select encryption type;
6086
encrypt OCTET STRING into packet.enc-part;
6087
6088
A.12. KRB_AP_REP verification
6089
6090
receive packet;
6091
6092
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6093
6094
6095
6096
6097
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6098
6099
if (packet.pvno != 5) then
6100
either process using other protocol spec
6101
or error_out(KRB_AP_ERR_BADVERSION);
6102
endif
6103
if (packet.msg-type != KRB_AP_REP) then
6104
error_out(KRB_AP_ERR_MSG_TYPE);
6105
endif
6106
cleartext := decrypt(packet.enc-part) using ticket's session key;
6107
if (decryption_error()) then
6108
error_out(KRB_AP_ERR_BAD_INTEGRITY);
6109
endif
6110
if (cleartext.ctime != authenticator.ctime) then
6111
error_out(KRB_AP_ERR_MUT_FAIL);
6112
endif
6113
if (cleartext.cusec != authenticator.cusec) then
6114
error_out(KRB_AP_ERR_MUT_FAIL);
6115
endif
6116
if (cleartext.subkey is present) then
6117
save cleartext.subkey for future use;
6118
endif
6119
if (cleartext.seq-number is present) then
6120
save cleartext.seq-number for future verifications;
6121
endif
6122
return(AUTHENTICATION_SUCCEEDED);
6123
6124
A.13. KRB_SAFE generation
6125
6126
collect user data in buffer;
6127
6128
/* assemble packet: */
6129
packet.pvno := protocol version; /* 5 */
6130
packet.msg-type := message type; /* KRB_SAFE */
6131
6132
body.user-data := buffer; /* DATA */
6133
if (using timestamp) then
6134
get system_time;
6135
body.timestamp, body.usec := system_time;
6136
endif
6137
if (using sequence numbers) then
6138
body.seq-number := sequence number;
6139
endif
6140
body.s-address := sender host addresses;
6141
if (only one recipient) then
6142
body.r-address := recipient host address;
6143
endif
6144
checksum.cksumtype := checksum type;
6145
compute checksum over body;
6146
checksum.checksum := checksum value; /* checksum.checksum */
6147
packet.cksum := checksum;
6148
packet.safe-body := body;
6149
6150
A.14. KRB_SAFE verification
6151
6152
receive packet;
6153
if (packet.pvno != 5) then
6154
either process using other protocol spec
6155
or error_out(KRB_AP_ERR_BADVERSION);
6156
endif
6157
if (packet.msg-type != KRB_SAFE) then
6158
6159
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6160
6161
6162
6163
6164
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6165
6166
error_out(KRB_AP_ERR_MSG_TYPE);
6167
endif
6168
if (packet.checksum.cksumtype is not both collision-proof
6169
and keyed) then
6170
error_out(KRB_AP_ERR_INAPP_CKSUM);
6171
endif
6172
if (safe_priv_common_checks_ok(packet)) then
6173
set computed_checksum := checksum(packet.body);
6174
if (computed_checksum != packet.checksum) then
6175
error_out(KRB_AP_ERR_MODIFIED);
6176
endif
6177
return (packet, PACKET_IS_GENUINE);
6178
else
6179
return common_checks_error;
6180
endif
6181
6182
A.15. KRB_SAFE and KRB_PRIV common checks
6183
6184
if (packet.s-address != O/S_sender(packet)) then
6185
/* O/S report of sender not who claims to have sent it */
6186
error_out(KRB_AP_ERR_BADADDR);
6187
endif
6188
if ((packet.r-address is present) and
6189
(packet.r-address != local_host_address)) then
6190
/* was not sent to proper place */
6191
error_out(KRB_AP_ERR_BADADDR);
6192
endif
6193
if (((packet.timestamp is present) and
6194
(not in_clock_skew(packet.timestamp,packet.usec))) or
6195
(packet.timestamp is not present and timestamp expected)) then
6196
error_out(KRB_AP_ERR_SKEW);
6197
endif
6198
if (repeated(packet.timestamp,packet.usec,packet.s-address)) then
6199
error_out(KRB_AP_ERR_REPEAT);
6200
endif
6201
6202
if (((packet.seq-number is present) and
6203
((not in_sequence(packet.seq-number)))) or
6204
(packet.seq-number is not present and sequence expected)) then
6205
error_out(KRB_AP_ERR_BADORDER);
6206
endif
6207
if (packet.timestamp not present and packet.seq-number
6208
not present) then
6209
error_out(KRB_AP_ERR_MODIFIED);
6210
endif
6211
6212
save_identifier(packet.{timestamp,usec,s-address},
6213
sender_principal(packet));
6214
6215
return PACKET_IS_OK;
6216
6217
A.16. KRB_PRIV generation
6218
6219
collect user data in buffer;
6220
6221
/* assemble packet: */
6222
packet.pvno := protocol version; /* 5 */
6223
packet.msg-type := message type; /* KRB_PRIV */
6224
6225
6226
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6227
6228
6229
6230
6231
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6232
6233
packet.enc-part.etype := encryption type;
6234
6235
body.user-data := buffer;
6236
if (using timestamp) then
6237
get system_time;
6238
body.timestamp, body.usec := system_time;
6239
endif
6240
if (using sequence numbers) then
6241
body.seq-number := sequence number;
6242
endif
6243
body.s-address := sender host addresses;
6244
if (only one recipient) then
6245
body.r-address := recipient host address;
6246
endif
6247
6248
encode body into OCTET STRING;
6249
6250
select encryption type;
6251
encrypt OCTET STRING into packet.enc-part.cipher;
6252
6253
A.17. KRB_PRIV verification
6254
6255
receive packet;
6256
if (packet.pvno != 5) then
6257
either process using other protocol spec
6258
or error_out(KRB_AP_ERR_BADVERSION);
6259
endif
6260
if (packet.msg-type != KRB_PRIV) then
6261
error_out(KRB_AP_ERR_MSG_TYPE);
6262
endif
6263
6264
cleartext := decrypt(packet.enc-part) using negotiated key;
6265
if (decryption_error()) then
6266
error_out(KRB_AP_ERR_BAD_INTEGRITY);
6267
endif
6268
6269
if (safe_priv_common_checks_ok(cleartext)) then
6270
return(cleartext.DATA, PACKET_IS_GENUINE_AND_UNMODIFIED);
6271
else
6272
return common_checks_error;
6273
endif
6274
6275
A.18. KRB_CRED generation
6276
6277
invoke KRB_TGS; /* obtain tickets to be provided to peer */
6278
6279
/* assemble packet: */
6280
packet.pvno := protocol version; /* 5 */
6281
packet.msg-type := message type; /* KRB_CRED */
6282
6283
for (tickets[n] in tickets to be forwarded) do
6284
packet.tickets[n] = tickets[n].ticket;
6285
done
6286
6287
packet.enc-part.etype := encryption type;
6288
6289
for (ticket[n] in tickets to be forwarded) do
6290
body.ticket-info[n].key = tickets[n].session;
6291
body.ticket-info[n].prealm = tickets[n].crealm;
6292
6293
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6294
6295
6296
6297
6298
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6299
6300
body.ticket-info[n].pname = tickets[n].cname;
6301
body.ticket-info[n].flags = tickets[n].flags;
6302
body.ticket-info[n].authtime = tickets[n].authtime;
6303
body.ticket-info[n].starttime = tickets[n].starttime;
6304
body.ticket-info[n].endtime = tickets[n].endtime;
6305
body.ticket-info[n].renew-till = tickets[n].renew-till;
6306
body.ticket-info[n].srealm = tickets[n].srealm;
6307
body.ticket-info[n].sname = tickets[n].sname;
6308
body.ticket-info[n].caddr = tickets[n].caddr;
6309
done
6310
6311
get system_time;
6312
body.timestamp, body.usec := system_time;
6313
6314
if (using nonce) then
6315
body.nonce := nonce;
6316
endif
6317
6318
if (using s-address) then
6319
body.s-address := sender host addresses;
6320
endif
6321
if (limited recipients) then
6322
body.r-address := recipient host address;
6323
endif
6324
6325
encode body into OCTET STRING;
6326
6327
select encryption type;
6328
encrypt OCTET STRING into packet.enc-part.cipher
6329
using negotiated encryption key;
6330
6331
A.19. KRB_CRED verification
6332
6333
receive packet;
6334
if (packet.pvno != 5) then
6335
either process using other protocol spec
6336
or error_out(KRB_AP_ERR_BADVERSION);
6337
endif
6338
if (packet.msg-type != KRB_CRED) then
6339
error_out(KRB_AP_ERR_MSG_TYPE);
6340
endif
6341
6342
cleartext := decrypt(packet.enc-part) using negotiated key;
6343
if (decryption_error()) then
6344
error_out(KRB_AP_ERR_BAD_INTEGRITY);
6345
endif
6346
if ((packet.r-address is present or required) and
6347
(packet.s-address != O/S_sender(packet)) then
6348
/* O/S report of sender not who claims to have sent it */
6349
error_out(KRB_AP_ERR_BADADDR);
6350
endif
6351
if ((packet.r-address is present) and
6352
(packet.r-address != local_host_address)) then
6353
/* was not sent to proper place */
6354
error_out(KRB_AP_ERR_BADADDR);
6355
endif
6356
if (not in_clock_skew(packet.timestamp,packet.usec)) then
6357
error_out(KRB_AP_ERR_SKEW);
6358
endif
6359
6360
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6361
6362
6363
6364
6365
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6366
6367
if (repeated(packet.timestamp,packet.usec,packet.s-address)) then
6368
error_out(KRB_AP_ERR_REPEAT);
6369
endif
6370
if (packet.nonce is required or present) and
6371
(packet.nonce != expected-nonce) then
6372
error_out(KRB_AP_ERR_MODIFIED);
6373
endif
6374
6375
for (ticket[n] in tickets that were forwarded) do
6376
save_for_later(ticket[n],key[n],principal[n],
6377
server[n],times[n],flags[n]);
6378
return
6379
6380
A.20. KRB_ERROR generation
6381
6382
/* assemble packet: */
6383
packet.pvno := protocol version; /* 5 */
6384
packet.msg-type := message type; /* KRB_ERROR */
6385
6386
get system_time;
6387
packet.stime, packet.susec := system_time;
6388
packet.realm, packet.sname := server name;
6389
6390
if (client time available) then
6391
packet.ctime, packet.cusec := client_time;
6392
endif
6393
packet.error-code := error code;
6394
if (client name available) then
6395
packet.cname, packet.crealm := client name;
6396
endif
6397
if (error text available) then
6398
packet.e-text := error text;
6399
endif
6400
if (error data available) then
6401
packet.e-data := error data;
6402
endif
6403
6404
B. Definition of common authorization data elements
6405
6406
This appendix contains the definitions of common authorization data
6407
elements. These common authorization data elements are recursivly
6408
defined, meaning the ad-data for these types will itself contain a
6409
sequence of authorization data whose interpretation is affected by the
6410
encapsulating element. Depending on the meaning of the encapsulating
6411
element, the encapsulated elements may be ignored, might be interpreted
6412
as issued directly by the KDC, or they might be stored in a separate
6413
plaintext part of the ticket. The types of the encapsulating elements
6414
are specified as part of the Kerberos specification because the
6415
behavior based on these values should be understood across
6416
implementations whereas other elements need only be understood by the
6417
applications which they affect.
6418
6419
In the definitions that follow, the value of the ad-type for the
6420
element will be specified in the subsection number, and the value of
6421
the ad-data will be as shown in the ASN.1 structure that follows the
6422
subsection heading.
6423
6424
B.1. If relevant
6425
6426
6427
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6428
6429
6430
6431
6432
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6433
6434
AD-IF-RELEVANT AuthorizationData
6435
6436
AD elements encapsulated within the if-relevant element are intended
6437
for interpretation only by application servers that understand the
6438
particular ad-type of the embedded element. Application servers that do
6439
not understand the type of an element embedded within the if-relevant
6440
element may ignore the uninterpretable element. This element promotes
6441
interoperability across implementations which may have local extensions
6442
for authorization.
6443
6444
B.2. Intended for server
6445
6446
AD-INTENDED-FOR-SERVER SEQUENCE {
6447
intended-server[0] SEQUENCE OF PrincipalName
6448
elements[1] AuthorizationData
6449
}
6450
6451
AD elements encapsulated within the intended-for-server element may be
6452
ignored if the application server is not in the list of principal names
6453
of intended servers. Further, a KDC issuing a ticket for an application
6454
server can remove this element if the application server is not in the
6455
list of intended servers.
6456
6457
Application servers should check for their principal name in the
6458
intended-server field of this element. If their principal name is not
6459
found, this element should be ignored. If found, then the encapsulated
6460
elements should be evaluated in the same manner as if they were present
6461
in the top level authorization data field. Applications and application
6462
servers that do not implement this element should reject tickets that
6463
contain authorization data elements of this type.
6464
6465
B.3. Intended for application class
6466
6467
AD-INTENDED-FOR-APPLICATION-CLASS SEQUENCE {
6468
intended-application-class[0] SEQUENCE OF GeneralString elements[1]
6469
AuthorizationData } AD elements encapsulated within the
6470
intended-for-application-class element may be ignored if the
6471
application server is not in one of the named classes of application
6472
servers. Examples of application server classes include "FILESYSTEM",
6473
and other kinds of servers.
6474
6475
This element and the elements it encapulates may be safely ignored by
6476
applications, application servers, and KDCs that do not implement this
6477
element.
6478
6479
B.4. KDC Issued
6480
6481
AD-KDCIssued SEQUENCE {
6482
ad-checksum[0] Checksum,
6483
i-realm[1] Realm OPTIONAL,
6484
i-sname[2] PrincipalName OPTIONAL,
6485
elements[3] AuthorizationData.
6486
}
6487
6488
ad-checksum
6489
A checksum over the elements field using a cryptographic checksum
6490
method that is identical to the checksum used to protect the
6491
ticket itself (i.e. using the same hash function and the same
6492
encryption algorithm used to encrypt the ticket) and using a key
6493
6494
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6495
6496
6497
6498
6499
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6500
6501
derived from the same key used to protect the ticket.
6502
i-realm, i-sname
6503
The name of the issuing principal if different from the KDC
6504
itself. This field would be used when the KDC can verify the
6505
authenticity of elements signed by the issuing principal and it
6506
allows this KDC to notify the application server of the validity
6507
of those elements.
6508
elements
6509
A sequence of authorization data elements issued by the KDC.
6510
The KDC-issued ad-data field is intended to provide a means for
6511
Kerberos principal credentials to embed within themselves privilege
6512
attributes and other mechanisms for positive authorization, amplifying
6513
the priveleges of the principal beyond what can be done using a
6514
credentials without such an a-data element.
6515
6516
This can not be provided without this element because the definition of
6517
the authorization-data field allows elements to be added at will by the
6518
bearer of a TGT at the time that they request service tickets and
6519
elements may also be added to a delegated ticket by inclusion in the
6520
authenticator.
6521
6522
For KDC-issued elements this is prevented because the elements are
6523
signed by the KDC by including a checksum encrypted using the server's
6524
key (the same key used to encrypt the ticket - or a key derived from
6525
that key). Elements encapsulated with in the KDC-issued element will be
6526
ignored by the application server if this "signature" is not present.
6527
Further, elements encapsulated within this element from a ticket
6528
granting ticket may be interpreted by the KDC, and used as a basis
6529
according to policy for including new signed elements within derivative
6530
tickets, but they will not be copied to a derivative ticket directly.
6531
If they are copied directly to a derivative ticket by a KDC that is not
6532
aware of this element, the signature will not be correct for the
6533
application ticket elements, and the field will be ignored by the
6534
application server.
6535
6536
This element and the elements it encapulates may be safely ignored by
6537
applications, application servers, and KDCs that do not implement this
6538
element.
6539
6540
B.5. And-Or
6541
6542
AD-AND-OR SEQUENCE {
6543
condition-count[0] INTEGER,
6544
elements[1] AuthorizationData
6545
}
6546
6547
When restrictive AD elements encapsulated within the and-or element are
6548
encountered, only the number specified in condition-count of the
6549
encapsulated conditions must be met in order to satisfy this element.
6550
This element may be used to implement an "or" operation by setting the
6551
condition-count field to 1, and it may specify an "and" operation by
6552
setting the condition count to the number of embedded elements.
6553
Application servers that do not implement this element must reject
6554
tickets that contain authorization data elements of this type.
6555
6556
B.6. Mandatory ticket extensions
6557
6558
AD-Mandatory-Ticket-Extensions Checksum
6559
6560
6561
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6562
6563
6564
6565
6566
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6567
6568
An authorization data element of type mandatory-ticket-extensions
6569
specifies a collision-proof checksum using the same hash algorithm used
6570
to protect the integrity of the ticket itself. This checksum will be
6571
calculated over an individual extension field. If there are more than
6572
one extension, multiple Mandatory-Ticket-Extensions authorization data
6573
elements may be present, each with a checksum for a different extension
6574
field. This restriction indicates that the ticket should not be
6575
accepted if a ticket extension is not present in the ticket for which
6576
the checksum does not match that checksum specified in the
6577
authorization data element. Application servers that do not implement
6578
this element must reject tickets that contain authorization data
6579
elements of this type.
6580
6581
B.7. Authorization Data in ticket extensions
6582
6583
AD-IN-Ticket-Extensions Checksum
6584
6585
An authorization data element of type in-ticket-extensions specifies a
6586
collision-proof checksum using the same hash algorithm used to protect
6587
the integrity of the ticket itself. This checksum is calculated over a
6588
separate external AuthorizationData field carried in the ticket
6589
extensions. Application servers that do not implement this element must
6590
reject tickets that contain authorization data elements of this type.
6591
Application servers that do implement this element will search the
6592
ticket extensions for authorization data fields, calculate the
6593
specified checksum over each authorization data field and look for one
6594
matching the checksum in this in-ticket-extensions element. If not
6595
found, then the ticket must be rejected. If found, the corresponding
6596
authorization data elements will be interpreted in the same manner as
6597
if they were contained in the top level authorization data field.
6598
6599
Note that if multiple external authorization data fields are present in
6600
a ticket, each will have a corresponding element of type
6601
in-ticket-extensions in the top level authorization data field, and the
6602
external entries will be linked to the corresponding element by their
6603
checksums.
6604
6605
C. Definition of common ticket extensions
6606
6607
This appendix contains the definitions of common ticket extensions.
6608
Support for these extensions is optional. However, certain extensions
6609
have associated authorization data elements that may require rejection
6610
of a ticket containing an extension by application servers that do not
6611
implement the particular extension. Other extensions have been defined
6612
beyond those described in this specification. Such extensions are
6613
described elswhere and for some of those extensions the reserved number
6614
may be found in the list of constants.
6615
6616
It is known that older versions of Kerberos did not support this field,
6617
and that some clients will strip this field from a ticket when they
6618
parse and then reassemble a ticket as it is passed to the application
6619
servers. The presence of the extension will not break such clients, but
6620
any functionaly dependent on the extensions will not work when such
6621
tickets are handled by old clients. In such situations, some
6622
implementation may use alternate methods to transmit the information in
6623
the extensions field.
6624
6625
C.1. Null ticket extension
6626
6627
6628
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6629
6630
6631
6632
6633
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6634
6635
TE-NullExtension OctetString -- The empty Octet String
6636
6637
The te-data field in the null ticket extension is an octet string of
6638
lenght zero. This extension may be included in a ticket granting ticket
6639
so that the KDC can determine on presentation of the ticket granting
6640
ticket whether the client software will strip the extensions field.
6641
6642
C.2. External Authorization Data
6643
6644
TE-ExternalAuthorizationData AuthorizationData
6645
6646
The te-data field in the external authorization data ticket extension
6647
is field of type AuthorizationData containing one or more authorization
6648
data elements. If present, a corresponding authorization data element
6649
will be present in the primary authorization data for the ticket and
6650
that element will contain a checksum of the external authorization data
6651
ticket extension.
6652
-----------------------------------------------------------------------
6653
[TM] Project Athena, Athena, and Kerberos are trademarks of the
6654
Massachusetts Institute of Technology (MIT). No commercial use of these
6655
trademarks may be made without prior written permission of MIT.
6656
6657
[1] Note, however, that many applications use Kerberos' functions only
6658
upon the initiation of a stream-based network connection. Unless an
6659
application subsequently provides integrity protection for the data
6660
stream, the identity verification applies only to the initiation of the
6661
connection, and does not guarantee that subsequent messages on the
6662
connection originate from the same principal.
6663
6664
[2] Secret and private are often used interchangeably in the
6665
literature. In our usage, it takes two (or more) to share a secret,
6666
thus a shared DES key is a secret key. Something is only private when
6667
no one but its owner knows it. Thus, in public key cryptosystems, one
6668
has a public and a private key.
6669
6670
[3] Of course, with appropriate permission the client could arrange
6671
registration of a separately-named prin- cipal in a remote realm, and
6672
engage in normal exchanges with that realm's services. However, for
6673
even small numbers of clients this becomes cumbersome, and more
6674
automatic methods as described here are necessary.
6675
6676
[4] Though it is permissible to request or issue tick- ets with no
6677
network addresses specified.
6678
6679
[5] The password-changing request must not be honored unless the
6680
requester can provide the old password (the user's current secret key).
6681
Otherwise, it would be possible for someone to walk up to an unattended
6682
ses- sion and change another user's password.
6683
6684
[6] To authenticate a user logging on to a local system, the
6685
credentials obtained in the AS exchange may first be used in a TGS
6686
exchange to obtain credentials for a local server. Those credentials
6687
must then be verified by a local server through successful completion
6688
of the Client/Server exchange.
6689
6690
[7] "Random" means that, among other things, it should be impossible to
6691
guess the next session key based on knowledge of past session keys.
6692
This can only be achieved in a pseudo-random number generator if it is
6693
based on cryptographic principles. It is more desirable to use a truly
6694
6695
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6696
6697
6698
6699
6700
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6701
6702
random number generator, such as one based on measurements of random
6703
physical phenomena.
6704
6705
[8] Tickets contain both an encrypted and unencrypted portion, so
6706
cleartext here refers to the entire unit, which can be copied from one
6707
message and replayed in another without any cryptographic skill.
6708
6709
[9] Note that this can make applications based on unreliable transports
6710
difficult to code correctly. If the transport might deliver duplicated
6711
messages, either a new authenticator must be generated for each retry,
6712
or the application server must match requests and replies and replay
6713
the first reply in response to a detected duplicate.
6714
6715
[10] This is used for user-to-user authentication as described in [8].
6716
6717
[11] Note that the rejection here is restricted to authenticators from
6718
the same principal to the same server. Other client principals
6719
communicating with the same server principal should not be have their
6720
authenticators rejected if the time and microsecond fields happen to
6721
match some other client's authenticator.
6722
6723
[12] In the Kerberos version 4 protocol, the timestamp in the reply was
6724
the client's timestamp plus one. This is not necessary in version 5
6725
because version 5 messages are formatted in such a way that it is not
6726
possible to create the reply by judicious message surgery (even in
6727
encrypted form) without knowledge of the appropriate encryption keys.
6728
6729
[13] Note that for encrypting the KRB_AP_REP message, the sub-session
6730
key is not used, even if present in the Authenticator.
6731
6732
[14] Implementations of the protocol may wish to provide routines to
6733
choose subkeys based on session keys and random numbers and to generate
6734
a negotiated key to be returned in the KRB_AP_REP message.
6735
6736
[15]This can be accomplished in several ways. It might be known
6737
beforehand (since the realm is part of the principal identifier), it
6738
might be stored in a nameserver, or it might be obtained from a
6739
configura- tion file. If the realm to be used is obtained from a
6740
nameserver, there is a danger of being spoofed if the nameservice
6741
providing the realm name is not authenti- cated. This might result in
6742
the use of a realm which has been compromised, and would result in an
6743
attacker's ability to compromise the authentication of the application
6744
server to the client.
6745
6746
[16] If the client selects a sub-session key, care must be taken to
6747
ensure the randomness of the selected sub- session key. One approach
6748
would be to generate a random number and XOR it with the session key
6749
from the ticket-granting ticket.
6750
6751
[17] This allows easy implementation of user-to-user authentication
6752
[8], which uses ticket-granting ticket session keys in lieu of secret
6753
server keys in situa- tions where such secret keys could be easily
6754
comprom- ised.
6755
6756
[18] For the purpose of appending, the realm preceding the first listed
6757
realm is considered to be the null realm ("").
6758
6759
[19] For the purpose of interpreting null subfields, the client's realm
6760
is considered to precede those in the transited field, and the server's
6761
6762
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6763
6764
6765
6766
6767
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6768
6769
realm is considered to follow them.
6770
6771
[20] This means that a client and server running on the same host and
6772
communicating with one another using the KRB_SAFE messages should not
6773
share a common replay cache to detect KRB_SAFE replays.
6774
6775
[21] The implementation of the Kerberos server need not combine the
6776
database and the server on the same machine; it is feasible to store
6777
the principal database in, say, a network name service, as long as the
6778
entries stored therein are protected from disclosure to and
6779
modification by unauthorized parties. However, we recommend against
6780
such strategies, as they can make system management and threat analysis
6781
quite complex.
6782
6783
[22] See the discussion of the padata field in section 5.4.2 for
6784
details on why this can be useful.
6785
6786
[23] Warning for implementations that unpack and repack data structures
6787
during the generation and verification of embedded checksums: Because
6788
any checksums applied to data structures must be checked against the
6789
original data the length of bit strings must be preserved within a data
6790
structure between the time that a checksum is generated through
6791
transmission to the time that the checksum is verified.
6792
6793
[24] It is NOT recommended that this time value be used to adjust the
6794
workstation's clock since the workstation cannot reliably determine
6795
that such a KRB_AS_REP actually came from the proper KDC in a timely
6796
manner.
6797
6798
[25] Note, however, that if the time is used as the nonce, one must
6799
make sure that the workstation time is monotonically increasing. If the
6800
time is ever reset backwards, there is a small, but finite, probability
6801
that a nonce will be reused.
6802
6803
[27] An application code in the encrypted part of a message provides an
6804
additional check that the message was decrypted properly.
6805
6806
[29] An application code in the encrypted part of a message provides an
6807
additional check that the message was decrypted properly.
6808
6809
[31] An application code in the encrypted part of a message provides an
6810
additional check that the message was decrypted properly.
6811
6812
[32] If supported by the encryption method in use, an initialization
6813
vector may be passed to the encryption procedure, in order to achieve
6814
proper cipher chaining. The initialization vector might come from the
6815
last block of the ciphertext from the previous KRB_PRIV message, but it
6816
is the application's choice whether or not to use such an
6817
initialization vector. If left out, the default initialization vector
6818
for the encryption algorithm will be used.
6819
6820
[33] This prevents an attacker who generates an incorrect AS request
6821
from obtaining verifiable plaintext for use in an off-line password
6822
guessing attack.
6823
6824
[35] In the above specification, UNTAGGED OCTET STRING(length) is the
6825
notation for an octet string with its tag and length removed. It is not
6826
a valid ASN.1 type. The tag bits and length must be removed from the
6827
confounder since the purpose of the confounder is so that the message
6828
6829
Neuman, Ts'o, Kohl Expires: 10 September, 2000
6830
6831
6832
6833
6834
INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-05 June 25, 1999
6835
6836
starts with random data, but the tag and its length are fixed. For
6837
other fields, the length and tag would be redundant if they were
6838
included because they are specified by the encryption type. [36] The
6839
ordering of the fields in the CipherText is important. Additionally,
6840
messages encoded in this format must include a length as part of the
6841
msg-seq field. This allows the recipient to verify that the message has
6842
not been truncated. Without a length, an attacker could use a chosen
6843
plaintext attack to generate a message which could be truncated, while
6844
leaving the checksum intact. Note that if the msg-seq is an encoding of
6845
an ASN.1 SEQUENCE or OCTET STRING, then the length is part of that
6846
encoding.
6847
6848
[37] In some cases, it may be necessary to use a different "mix-in"
6849
string for compatibility reasons; see the discussion of padata in
6850
section 5.4.2.
6851
6852
[38] In some cases, it may be necessary to use a different "mix-in"
6853
string for compatibility reasons; see the discussion of padata in
6854
section 5.4.2.
6855
6856
[39] A variant of the key is used to limit the use of a key to a
6857
particular function, separating the functions of generating a checksum
6858
from other encryption performed using the session key. The constant
6859
F0F0F0F0F0F0F0F0 was chosen because it maintains key parity. The
6860
properties of DES precluded the use of the complement. The same
6861
constant is used for similar purpose in the Message Integrity Check in
6862
the Privacy Enhanced Mail standard.
6863
6864
[40] This error carries additional information in the e- data field.
6865
The contents of the e-data field for this message is described in
6866
section 5.9.1.
Loggerhead is a web-based interface for Breezy
Version: 2.0.1