3
NETWORK WORKING GROUP L. Zhu
4
Internet-Draft P. Leach
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Updates: 4120 (if approved) K. Jaganathan
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Intended status: Standards Track Microsoft Corporation
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Expires: September 3, 2007 March 2, 2007
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Anonymity Support for Kerberos
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draft-ietf-krb-wg-anon-03
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By submitting this Internet-Draft, each author represents that any
16
applicable patent or other IPR claims of which he or she is aware
17
have been or will be disclosed, and any of which he or she becomes
18
aware will be disclosed, in accordance with Section 6 of BCP 79.
20
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-
25
Internet-Drafts are draft documents valid for a maximum of six months
26
and may be updated, replaced, or obsoleted by other documents at any
27
time. It is inappropriate to use Internet-Drafts as reference
28
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
31
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 September 3, 2007.
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Copyright (C) The IETF Trust (2007).
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This document defines extensions to the Kerberos protocol for the
45
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 . . . . . . . . . . . . . . . . . 7
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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 . . . . . . . . . . . . . . . . . . . . . 9
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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_RESRVED [KRBNAM], and the value of the
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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 TBA (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 request-anonymous KDC option is defined as bit TBA (with the
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first bit being bit 0) in the KDCOptions:
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KDCOptions ::= KerberosFlags
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-- request-anonymous(TBA)
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-- KDCOptions and KerberosFlags are defined in [RFC4120]
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As described in Section 4, the request-anonymous KDC option is set to
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request an anonymous ticket.
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4. Protocol Description
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In order to request an anonymous ticket, the client sets the request-
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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
236
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
239
anonymous, or if the client in the AS request is anonymous, the
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request-anonymous KDC option MUST be set in the request. Otherwise,
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the KDC MUST return a KRB-ERROR message with the code
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KDC_ERR_BADOPTION [RFC4120], and there is no accompanying e-data
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defined in this document.
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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
248
signature and the client's public key (such as the verification of
249
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
252
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
254
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
261
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
272
(the reply is anonymous), the client MUST reject the returned ticket
273
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,
285
the reply key is selected as normal according to Section 3.1.3 of
288
For the TGS exchange, the reply key is selected as normal according
289
to Section 3.3.3 of [RFC4120].
291
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,
294
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
301
During the TGS request, when propagating authorization data, care
302
MUST be taken by the TGS to ensure that the client confidentiality is
303
not violated. If a anonymous ticket is returned, any authorization
304
element that may reveal the client's identity MUST be removed. The
305
authentication attempt MUST be rejected if there is an authorization
306
element that is intended to restrict the use of the ticket thus
307
cannot be removed or the local policy prevents the removal of an
308
authorization element, and this rule MUST be applied to all critical
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and optional authorization data. An optional authorization element
310
unknown by the TGS MUST be removed if it does not potentially convey
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any rights or limit the rights otherwise conveyed in the ticket. If
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there is a critical unknown authorization element, unless this
313
element is encapsulated in a known authorization data element
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amending the criticality of the elements it contains, authentication
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MUST fail according to [RFC4120]. If it is inappropriate to remove
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an authorization element from the TGS request in order to produce an
317
anonymous ticket, the KDC MUST return an error message with the code
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KDC_ERR_POLICY [RFC4120], and there is no accompanying e-data defined
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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
324
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
326
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.
346
If the client is anonymous and the KDC does not have a key to encrypt
347
the reply (this can happen when, for example, the KDC does not
348
support PKINIT [RFC4556]), the KDC MUST return an error message with
349
the code KDC_ERR_NULL_KEY [RFC4120] and there is no accompanying
350
e-data defined in this document.
352
If a client requires anonymous communication then the client MUST
353
check to make sure that the ticket in the reply is actually anonymous
354
by checking the presence of the anonymous ticket flag. This is
355
because KDCs ignore unknown KDC options. A KDC that does not
356
understand the request-anonymous KDC option will not return an error,
357
but will instead return a normal ticket.
359
The subsequent client and server communications then proceed as
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described in [RFC4120].
362
A server accepting an anonymous service ticket may assume that
363
subsequent requests using the same ticket originate from the same
364
client. Requests with different tickets are likely to originate from
368
5. GSS-API Implementation Notes
370
At the GSS-API [RFC2743] level, the use of an anonymous principal by
371
the initiator/client requires the initiator/client to assert the
372
"anonymous" flag when calling GSS_Init_Sec_Context().
374
GSS-API does not know or define "anonymous credentials", so the
375
(printable) name of the anonymous principal will rarely be used by or
376
relevant for the initiator/client. The printable name is relevant
377
for the acceptor/server when performing an authorization decision
378
based on the initiator name that is returned from the acceptor side
379
upon the successful security context establishment.
381
A GSS-API initiator MUST carefully check the resulting context
382
attributes from the initial call to GSS_Init_Sec_Context() when
383
requesting anonymity, because (as in the GSS-API tradition and for
384
backwards compatibility) anonymity is just another optional context
385
attribute. It could be that the mechanism doesn't recognize the
386
attribute at all or that anonymity is not available for some other
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reasons -- and in that case the initiator must NOT send the initial
396
security context token to the acceptor, because it will likely reveal
397
the initiators identity to the acceptor, something that can rarely be
400
GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
401
represent the anonymous identity. In addition, Section 2.1.1 of
402
[RFC1964] defines the single string representation of a Kerberos
403
principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. For
404
the anonymous principals, the name component within the exportable
405
name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm
406
name according to Section 2.1.1 of [RFC1964]. Note that in this
407
specification only the client/initiator can be anonymous.
409
Portable initiators are RECOMMENDED to use default credentials
410
whenever possible, and request anonymity only through the input
411
anon_req_flag [RFC2743] to GSS_Init_Sec_Context().
414
6. Security Considerations
416
Since KDCs ignore unknown options [RFC4120], a client requiring
417
anonymous communication needs to make sure that the ticket is
418
actually anonymous. This is because a KDC that that does not
419
understand the anonymous option would not return an anonymous ticket.
421
By using the mechanism defined in this specification, the client does
422
not reveal its identity to the server but its identity may be
423
revealed to the KDC of the server principal (when the server
424
principal is in a different realm than that of the client), and any
425
KDC on the cross-realm authentication path. The Kerberos client MUST
426
verify the ticket being used is indeed anonymous before communicating
427
with the server, otherwise the client's identity may be revealed
430
In cases where specific server principals must not have access to the
431
client's identity (for example, an anonymous poll service), the KDC
432
can define server principal specific policy that insure any normal
433
service ticket can NEVER be issued to any of these server principals.
435
If the KDC that issued an anonymous ticket were to maintain records
436
of the association of identities to an anonymous ticket, then someone
437
obtaining such records could breach the anonymity. Additionally, the
438
implementations of most (for now all) KDC's respond to requests at
439
the time that they are received. Traffic analysis on the connection
440
to the KDC will allow an attacker to match client identities to
441
anonymous tickets issued. Because there are plaintext parts of the
442
tickets that are exposed on the wire, such matching by a third party
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observer is relatively straightforward.
456
Clifford Neuman contributed the core notions of this document.
458
Ken Raeburn reviewed the document and provided suggestions for
461
Martin Rex wrote the text for GSS-API considerations.
463
Nicolas Williams reviewed the GSS-API considerations section and
464
suggested ideas for improvements.
466
Sam Hartman and Nicolas Williams were great champions of this work.
468
In addition, the following individuals made significant
469
contributions: Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love
470
Hoernquist Aestrand, and Jeffery Hutzelman.
473
8. IANA Considerations
475
Section 3 defines the anonymous Kerberos name and the anonymous
476
Kerberos realm based on [KRBNAM]. The IANA registry for [KRBNAM]
477
need to be updated to add references to this document.
480
9. Normative References
482
[KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints",
483
draft-ietf-krb-wg-naming, work in progress.
485
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
488
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
489
Requirement Levels", BCP 14, RFC 2119, March 1997.
491
[RFC2743] Linn, J., "Generic Security Service Application Program
492
Interface Version 2, Update 1", RFC 2743, January 2000.
494
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
497
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
498
Kerberos Network Authentication Service (V5)", RFC 4120,
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[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
509
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
511
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
512
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
518
Microsoft Corporation
523
Email: lzhu@microsoft.com
527
Microsoft Corporation
532
Email: paulle@microsoft.com
536
Microsoft Corporation
541
Email: karthikj@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
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587
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588
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590
found in BCP 78 and BCP 79.
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593
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595
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596
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597
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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
609
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