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NETWORK WORKING GROUP L. Zhu
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Internet-Draft P. Leach
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Updates: 4120 (if approved) Microsoft Corporation
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Intended status: Standards Track July 7, 2007
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Expires: January 8, 2008
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Anonymity Support for Kerberos
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draft-ietf-krb-wg-anon-04
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By submitting this Internet-Draft, each author represents that any
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applicable patent or other IPR claims of which he or she is aware
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have been or will be disclosed, and any of which he or she becomes
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aware will be disclosed, in accordance with Section 6 of BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF), its areas, and its working groups. Note that
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other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
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and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them 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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The list of Internet-Draft Shadow Directories can be accessed at
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http://www.ietf.org/shadow.html.
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This Internet-Draft will expire on January 8, 2008.
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Copyright (C) The IETF Trust (2007).
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This document defines extensions to the Kerberos protocol for the
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Kerberos client to authenticate the Kerberos Key Distribution Center
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and the Kerberos server, without revealing the client's identity.
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These extensions can be used to secure communication between the
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anonymous client and the server.
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
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2. Conventions Used in This Document . . . . . . . . . . . . . . 3
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3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
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4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 4
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5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 8
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6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
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7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
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8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
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9. Normative References . . . . . . . . . . . . . . . . . . . . . 10
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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Intellectual Property and Copyright Statements . . . . . . . . . . 11
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In certain situations, the Kerberos [RFC4120] client may wish to
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authenticate a server and/or protect communications without revealing
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its own identity. For example, consider an application which
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provides read access to a research database, and which permits
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queries by arbitrary requestors. A client of such a service might
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wish to authenticate the service, to establish trust in the
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information received from it, but might not wish to disclose its
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identity to the service for privacy reasons.
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Extensions to [RFC4120] are specified in this document by which a
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client can authenticate the Key Distribution Center (KDC) and request
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an anonymous ticket. The client can use the anonymous ticket to
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authenticate the server and protect subsequent client-server
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communications. These extensions provide Kerberos with functional
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equivalence to Transport Layer Security (TLS) [RFC4346].
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By using the extensions defined in this specification, the client may
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reveal its identity in its initial request to its own KDC, but it can
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remain anonymous thereafter to KDCs on the cross-realm authentication
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path, and to the server with which it communicates.
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2. Conventions Used in This Document
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
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The anonymous Kerberos realm name is defined as a well-known realm
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name based on [KRBNAM]. The value is the literal "WELLKNOWN:
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ANONYMOUS". An anonymous Kerberos realm name MUST NOT be present in
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the transited field [RFC4120] of a ticket.
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The anonymous Kerberos principal name is defined as a well-known
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Kerberos principal name based on [KRBNAM]. The value of the name-
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type field [RFC4120] is KRB_NT_WELLKNOWN [KRBNAM], and the value of
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the name-string field [RFC4120] is a sequence of two KerberosString
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components: "WELLKNOWN", "ANONYMOUS".
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Note that in this specification, the anonymous principal name and
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realm are only applicable to the client in Kerberos messages, the
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server MUST NOT be anonymous in any Kerberos message.
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The anonymous ticket flag is defined as bit 14 (with the first bit
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being bit 0) in the TicketFlags:
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TicketFlags ::= KerberosFlags
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-- TicketFlags and KerberosFlags are defined in [RFC4120]
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An anonymous ticket is a ticket that has all of the following
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o The cname field [RFC4120] contains the anonymous Kerberos
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o The crealm field [RFC4120] contains the client's realm name, or
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the name of the realm that issued the initial ticket for the
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client principal, or the anonymous realm name.
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o The anonymous ticket contains no information that can reveal the
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client's identity. However the ticket may contain the client
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realm, intermediate realms on the client's authentication path,
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and authorization data that may provide information related to the
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client's identity. For example, an anonymous principal that is
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identifiable only within a particular group of users can be
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implemented using authorization data and such authorization data,
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if included in the anonymous ticket, shall disclose the client's
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membership of that group.
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o The anonymous ticket flag is set.
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The anonymous KDC option is defined as bit 14 (with the first bit
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being bit 0) in the KDCOptions:
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KDCOptions ::= KerberosFlags
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-- KDCOptions and KerberosFlags are defined in [RFC4120]
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As described in Section 4, the anonymous KDC option is set to request
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4. Protocol Description
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In order to request an anonymous ticket, the client sets the
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anonymous KDC option in an Authentication Exchange (AS) or Ticket
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Granting Service (TGS) request [RFC4120]. The client can request an
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anonymous Ticket Granting Ticket (TGT) based on a normal TGT. Unless
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otherwise specified, the client can obtain an anonymous ticket with
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the anonymous realm name only by requesting an anonymous ticket in an
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AS exchange with the client realm set as anonymous in the request.
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If the client wishes to authenticate the KDC anonymously, it sets the
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client name as anonymous in the AS exchange and provides a
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PA_PK_AS_REQ pre-authentication data [RFC4556] where both the
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signerInfos field and the certificates field of the SignedData
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[RFC3852] of the PA_PK_AS_REQ are empty. Because the anonymous
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client does not have an associated asymmetric key pair, the client
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MUST choose the Diffie-Hellman key agreement method by filling in the
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Diffie-Hellman domain parameters in the clientPublicValue [RFC4556].
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If the ticket in the PA-TGS-REQ [RFC4120] of the TGS request is
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anonymous, or if the client in the AS request is anonymous, the
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anonymous KDC option MUST be set in the request. Otherwise, the KDC
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MUST return a KRB-ERROR message with the code KDC_ERR_BADOPTION
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[RFC4120], and there is no accompanying e-data defined in this
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Upon receiving the AS request with a PA_PK_AS_REQ [RFC4556] from the
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anonymous client, the KDC processes the request according to Section
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3.1.2 of [RFC4120]. The KDC skips the checks for the client's
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signature and the client's public key (such as the verification of
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the binding between the client's public key and the client name), but
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performs otherwise-applicable checks, and proceeds as normal
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according to [RFC4556]. For example, the AS MUST check if the
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client's Diffie-Hellman domain parameters are acceptable. The
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Diffie-Hellman key agreement method MUST be used and the reply key is
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derived according to Section 3.2.3.1 of [RFC4556]. If the
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clientPublicValue is not present in the request, the KDC MUST return
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a KRB-ERROR [RFC4120] with the code
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KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556] and there is no
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accompanying e-data. If all goes well, an anonymous ticket is
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generated according to Section 3.1.3 of [RFC4120] and a PA_PK_AS_REP
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[RFC4556] pre-authentication data is included in the KDC reply
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according to [RFC4556]. If the KDC does not have an asymmetric key
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pair, it MAY reply anonymously or reject the authentication attempt.
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If the KDC replies anonymously, both the signerInfos field and the
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certificates field of the SignedData [RFC3852] of PA_PK_AS_REP in the
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reply are empty. The server name in the anonymous KDC reply contains
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Upon receipt of the KDC reply that contains an anonymous ticket and a
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PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then
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authenticate the KDC based on the KDC's signature in the
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PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply
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(the reply is anonymous), the client MUST reject the returned ticket
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if it cannot authenticate the KDC otherwise.
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The client can use the client keys to mutually authenticate with the
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KDC, request an anonymous TGT in the AS request. And in that case,
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the reply key is selected as normal according to Section 3.1.3 of
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For the TGS exchange, the reply key is selected as normal according
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to Section 3.3.3 of [RFC4120].
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When policy allows, the KDC issues an anonymous ticket. Based on
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local policy, the client realm in the anonymous ticket can be the
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anonymous realm name or the realm of the KDC. However, in all cases,
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the client name and the client realm in the EncKDCRepPart of the
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reply [RFC4120] MUST match with the corresponding client name and the
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client realm of the anonymous ticket in the reply. The client MUST
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use the client name and the client realm returned in the
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EncKDCRepPart in subsequent message exchanges when using the obtained
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When propagating authorization data in the ticket or in the enc-
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authorization-data field [RFC4120] of the request, the TGS MUST
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ensure that the client confidentiality is not violated in the
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returned anonymous ticket. The TGS MUST process the authorization
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data recursively according to Section 5.2.6 of [RFC4120] beyond the
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container levels such that all embedded authorization elements are
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interpreted. Identity-based authorization data SHOULD NOT be present
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in an anonymous ticket in that it typically reveals the client's
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identity. The specification of a new authorization data type MUST
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specify the processing rules of the authorization data when an
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anonymous ticket is returned. If there is no processing rule defined
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for an authorization data element or the authorization data element
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is unknown, the TGS MUST process it when an anonymous ticket is
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o If the authorization data element may reveal the client's
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identity, it MUST be removed unless otherwise specified.
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o If the authorization data element is intended to restrict the use
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of the ticket or limit the rights otherwise conveyed in the
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ticket, it cannot be removed in order to hide the client's
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identity. In this case, the authentication attempt MUST be
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rejected, and the KDC MUST return an error message with the code
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KDC_ERR_POLICY [RFC4120]. There is no accompanying e-data defined
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in this document. Note this is applicable to both critical and
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optional authorization data.
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o If the authorization data element is unknown, the TGS MAY remove
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it, or transfer it into the returned anonymous ticket, or reject
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the authentication attempt, based on local policy for that
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authorization data type unless otherwise specified. If there is
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no policy defined for a given unknown authorization data type, the
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authentication MUST be rejected. The error code is KDC_ERR_POLICY
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when the authentication is rejected.
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The AD-INITIAL-VERIFIED-CAS authorization data [RFC4556] MAY be
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removed from an anonymous ticket based on local policy of the TGS.
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The TGS MUST add the name of the previous realm according to Section
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3.3.3.2 of [RFC4120]. If the client's realm is the anonymous realm,
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the abbreviation forms [RFC4120] that build on the preceding name
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cannot be used at the start of the transited encoding. The null-
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subfield form (e.g., encoding ending with ",") [RFC4120] could not be
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used next to the anonymous realm that can potentially be at the
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beginning where the client realm is filled in.
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The KDC fills out the authtime field of the anonymous ticket in the
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reply as follows: If the anonymous ticket is returned in an AS
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exchange, the authtime field of the ticket contains the request time.
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If the anonymous ticket is returned in a TGS exchange, the authtime
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field contains the authtime of the ticket in the PA-TGS-REQ pre-
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authentication data [RFC4120]. An anonymous ticket can be renewed,
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and the authtime field of a renewed ticket is the authtime in the
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anonymous ticket on which the renewed ticket was based.
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If the client is anonymous and the KDC does not have a key to encrypt
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the reply (this can happen when, for example, the KDC does not
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support PKINIT [RFC4556]), the KDC MUST return an error message with
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the code KDC_ERR_NULL_KEY [RFC4120] and there is no accompanying
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e-data defined in this document.
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If a client requires anonymous communication then the client MUST
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check to make sure that the ticket in the reply is actually anonymous
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by checking the presence of the anonymous ticket flag. This is
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because KDCs ignore unknown KDC options. A KDC that does not
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understand the anonymous KDC option will not return an error, but
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will instead return a normal ticket.
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The subsequent client and server communications then proceed as
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described in [RFC4120].
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A server accepting an anonymous service ticket may assume that
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subsequent requests using the same ticket originate from the same
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client. Requests with different tickets are likely to originate from
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5. GSS-API Implementation Notes
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At the GSS-API [RFC2743] level, the use of an anonymous principal by
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the initiator/client requires the initiator/client to assert the
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"anonymous" flag when calling GSS_Init_Sec_Context().
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GSS-API does not know or define "anonymous credentials", so the
402
(printable) name of the anonymous principal will rarely be used by or
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relevant for the initiator/client. The printable name is relevant
404
for the acceptor/server when performing an authorization decision
405
based on the initiator name that is returned from the acceptor side
406
upon the successful security context establishment.
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A GSS-API initiator MUST carefully check the resulting context
409
attributes from the initial call to GSS_Init_Sec_Context() when
410
requesting anonymity, because (as in the GSS-API tradition and for
411
backwards compatibility) anonymity is just another optional context
412
attribute. It could be that the mechanism doesn't recognize the
413
attribute at all or that anonymity is not available for some other
414
reasons -- and in that case the initiator must NOT send the initial
415
security context token to the acceptor, because it will likely reveal
416
the initiators identity to the acceptor, something that can rarely be
419
GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
420
represent the anonymous identity. In addition, Section 2.1.1 of
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[RFC1964] defines the single string representation of a Kerberos
422
principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. For
423
the anonymous principals, the name component within the exportable
424
name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm
425
name according to Section 2.1.1 of [RFC1964]. Note that in this
426
specification only the client/initiator can be anonymous.
428
Portable initiators are RECOMMENDED to use default credentials
429
whenever possible, and request anonymity only through the input
430
anon_req_flag [RFC2743] to GSS_Init_Sec_Context().
433
6. Security Considerations
435
Since KDCs ignore unknown options [RFC4120], a client requiring
436
anonymous communication needs to make sure that the ticket is
437
actually anonymous. This is because a KDC that that does not
438
understand the anonymous option would not return an anonymous ticket.
440
By using the mechanism defined in this specification, the client does
441
not reveal its identity to the server but its identity may be
442
revealed to the KDC of the server principal (when the server
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principal is in a different realm than that of the client), and any
452
KDC on the cross-realm authentication path. The Kerberos client MUST
453
verify the ticket being used is indeed anonymous before communicating
454
with the server, otherwise the client's identity may be revealed
457
In cases where specific server principals must not have access to the
458
client's identity (for example, an anonymous poll service), the KDC
459
can define server principal specific policy that insure any normal
460
service ticket can NEVER be issued to any of these server principals.
462
If the KDC that issued an anonymous ticket were to maintain records
463
of the association of identities to an anonymous ticket, then someone
464
obtaining such records could breach the anonymity. Additionally, the
465
implementations of most (for now all) KDC's respond to requests at
466
the time that they are received. Traffic analysis on the connection
467
to the KDC will allow an attacker to match client identities to
468
anonymous tickets issued. Because there are plaintext parts of the
469
tickets that are exposed on the wire, such matching by a third party
470
observer is relatively straightforward.
475
JK Jaganathan helped editing early revisions of this document.
477
Clifford Neuman contributed the core notions of this document.
479
Ken Raeburn reviewed the document and provided suggestions for
482
Martin Rex wrote the text for GSS-API considerations.
484
Nicolas Williams reviewed the GSS-API considerations section and
485
suggested ideas for improvements.
487
Sam Hartman and Nicolas Williams were great champions of this work.
489
In addition, the following individuals made significant
490
contributions: Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love
491
Hoernquist Aestrand, and Jeffery Hutzelman.
494
8. IANA Considerations
496
Section 3 defines the anonymous Kerberos name and the anonymous
497
Kerberos realm based on [KRBNAM]. The IANA registry for [KRBNAM]
498
need to be updated to add references to this document.
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9. Normative References
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[KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints",
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draft-ietf-krb-wg-naming, work in progress.
512
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
516
Requirement Levels", BCP 14, RFC 2119, March 1997.
518
[RFC2743] Linn, J., "Generic Security Service Application Program
519
Interface Version 2, Update 1", RFC 2743, January 2000.
521
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
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[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
525
Kerberos Network Authentication Service (V5)", RFC 4120,
528
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
529
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
531
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
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Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
538
Microsoft Corporation
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Email: lzhu@microsoft.com
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Microsoft Corporation
552
Email: paulle@microsoft.com
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Full Copyright Statement
566
Copyright (C) The IETF Trust (2007).
568
This document is subject to the rights, licenses and restrictions
569
contained in BCP 78, and except as set forth therein, the authors
570
retain all their rights.
572
This document and the information contained herein are provided on an
573
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
574
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
575
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
576
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
577
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
578
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
581
Intellectual Property
583
The IETF takes no position regarding the validity or scope of any
584
Intellectual Property Rights or other rights that might be claimed to
585
pertain to the implementation or use of the technology described in
586
this document or the extent to which any license under such rights
587
might or might not be available; nor does it represent that it has
588
made any independent effort to identify any such rights. Information
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on the procedures with respect to rights in RFC documents can be
590
found in BCP 78 and BCP 79.
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Copies of IPR disclosures made to the IETF Secretariat and any
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such proprietary rights by implementers or users of this
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specification can be obtained from the IETF on-line IPR repository at
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http://www.ietf.org/ipr.
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600
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601
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602
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608
Funding for the RFC Editor function is provided by the IETF
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