NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Updates: 4120 (if approved)                                K. Jaganathan
Expires: December 5, 2006                          Microsoft Corporation
                                                            June 3, 2006

                     Anonymity Support for Kerberos

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document defines the use of anonymous Kerberos tickets for the
   purpose of authenticating the servers and enabling secure
   communication between a client and a server, without identifying the
   client to the server.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  5
   5.  GSS-API Implementation Notes . . . . . . . . . . . . . . . . .  6
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   9.  Normative References . . . . . . . . . . . . . . . . . . . . .  7
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Introduction

   In certain situations or environments, the Kerberos [RFC4120] client
   may wish to authenticate a server and/or protect communications
   without revealing its own identity.  For example, consider an
   application which provides read access to a research database, and
   which permits queries by arbitrary requestors.  A client of such a
   service might wish to authenticate the service, to establish trust in
   the information received from it, but might not wish to disclose its
   identity to the service for privacy reasons.

   To accomplish this, a Kerberos mechanism is specified in this
   document by which a client requests an anonymous ticket and use that
   to authenticate the server and secure subsequent client-server
   communications.  This provides Kerberos with functional equivalence
   to TLS [RFC2246] in environments where Kerberos is a more attractive
   authentication mechanism.

   Using this mechanism, the client has to reveal its identity in its
   initial request to its own Key Distribution Center (KDC) [RFC4120],
   and then it can remain anonymous thereafter to KDCs on the cross-
   realm authentication path, if any, and to the server with which it

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Definitions

   An anonymous ticket is a ticket that has all of the following

   o  The client's principal name is the anonymous Kerberos principal
      name.  The anonymous Kerberos principal name is defined as
      follows: it is a reserved Kerberos principal name as defined in
      [KRBNAM], the name-type is KRB_NT_RESRVED [KRBNAM], and the name-
      string is a sequence of two KerberosString components: "RESERVED",

   o  The client's realm name is the anonymous kerberos realm name.  The
      anonymous Kerberos realm name is defined as follows: it is a
      reserved realm name as defined in [KRBNAM] and the realm name is
      the literal "RESERVED:ANONYMOUS".

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   o  The authtime field in the ticket is set to the time of the ticket
      request, not the time of the initial authentication for the
      principal who has made the request.

   o  The transited field [RFC4120] can either contain the client's
      authentication path or contain the anonymous authentication path
      defined as follows: the tr-type field of the transited field is
      NO-TRANSITED-INFO (as defined later in this section) and the
      contents field is an empty OCTET STRING.  If a TGS request
      contains an anonymous ticket with a "normal" authentication path
      (i.e. the transited field does not contain the anonymous
      authentication path as defined above), then the reply ticket, if
      any, MUST NOT contain the anonymous authentication path.  For
      application servers, no transited policy is defined for the
      anonymous authentication path, but all of the transited checks
      would still apply if an anonymous ticket contains a "normal"
      authentication path.  Note that the "normal" authentication path
      in an anonymous ticket can be a partial path, thus it may not be
      sufficient to identify the originating client realm.

   o  It contains no information that can reveal the client's identity
      other than, at most, the client's realm or the realm(s) on the
      authentication path.

   o  The anonymous ticket flag (as defined later in this section) is

   The anonymous ticket flag is defined as bit 14 (with the first bit
   being bit 0) in the TicketFlags:

           TicketFlags     ::= KerberosFlags
             -- anonymous(14)
             -- TicketFlags and KerberosFlags are defined in [RFC4120]

   The anonymous ticket flag MUST NOT be set by implementations of this
   specification if the ticket is not an anonymous ticket as defined in
   this section.

   The request-anonymous KDC option is defined as bit 14 (with the first
   bit being bit 0) in the KDCOptions:

           KDCOptions      ::= KerberosFlags
             -- request-anonymous(14)
             -- KDCOptions and KerberosFlags are defined in [RFC4120]

   The anonymous transited encoding type is defined as follows:

           NO-TRANSITED-INFO    0

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   This transited encoding type indicates that there is no information
   available about the authentication path.

   Note that the server principal name and the server realm in a cross-
   realm referral TGT are not dependent on whether the client is the
   anonymous principal or not.

4.  Protocol Description

   In order to request an anonymous ticket, the client sets the request-
   anonymous KDC option in an AS or TGS request [RFC4120].  Note that if
   the service ticket in the PA-TGS-REQ [RFC4120] is anonymous, the
   request-anonymous KDC option MUST be set in the request.

   When policy allows, the KDC issues an anonymous ticket.  The KDC that
   implements this specification MUST NOT carry information that can
   reveal the client's identity, from the TGS request into the returned
   anonymous ticket.

   It should be noted that unless otherwise specified by this document
   the client principal name and the client realm in the Kerberos
   messages [RFC4120] should be the client name and client realm that
   can uniquely identify the client principal to the KDC, not the
   anonymous client principal name and the empty realm name.  For
   example, the client name and realm in the request body and the
   EncKDCRepPart of the reply [RFC4120] are identifiers of the client
   principal.  In other words, the client name and client realm in the
   EncKDCRepPart does not match with that of the returned anonymous

   If either local policy prohibits issuing of anonymous tickets or it
   is inappropriate to remove information (such as restrictions) from
   the TGS request in order to produce an anonymous ticket, the KDC MUST
   return an error message with the code KDC_ERR_POLICY [RFC4120].

   If a client requires anonymous communication then the client should
   check to make sure that the resulting ticket is actually anonymous by
   checking the presence of the anonymous ticket flag.  Because KDCs
   ignore unknown KDC options, a KDC that does not understand the
   request-anonymous KDC option will not return an error, but will
   instead return a normal ticket.

   The subsequent client and server communications then proceed as
   described in [RFC4120].  The client principal name in the
   Authenticator of the KRB_AP_REQ MUST be the anonymous client
   principal name and the client realm of the Authenticator MUST be an
   empty KerberosString [RFC4120].

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   A server accepting such an anonymous service ticket may assume that
   subsequent requests using the same ticket originate from the same
   client.  Requests with different tickets are likely to originate from
   different clients.

   Interoperability and backward-compatibility notes: the KDC is given
   the task of rejecting a request for an anonymous ticket when the
   anonymous ticket is not acceptable by the server.

5.  GSS-API Implementation Notes

   At the GSS-API [RFC2743] level, the use of an anonymous principal by
   the initiator/client requires a software change of the initiator/
   client software (to assert the "anonymous" flag when calling

   GSS-API does not know or define "anonymous credentials", so the
   (printable) name of the anonymous principal will rarely be used by or
   relevant for the initator/client.  The printable name is relevant for
   the acceptor/server when performing an authorization decision based
   on the name that pops up from GSS_Accept_Sec_Context() upon
   successful security context establishment.

   A GSS-API initiator MUST carefully check the resulting context
   attributes from the initial call to GSS_Init_Sec_Context() when
   requesting anonymity, because (as in the GSS-API tradition and for
   backwards compatibility) anonymity is just another optional context
   attribute.  It could be that the mechanism doesn't recognize the
   attribute at all or that anonymity is not available for some other
   reasons -- and in that case the initiator must NOT send the initial
   security context token to the acceptor, because it will likely reveal
   the initiators identity to the acceptor, something that can rarely be

   GSS-API defines name_type GSS_C_NT_ANONYMOUS [RFC2743] to represent
   an anonymous identity.  In addition, according to Section 2.1.1 of
   [RFC1964] the string representation of the anonymous client principal
   GSS_KRB5_NT_PRINCIPAL_NAME.  Implementations conforming to this
   specification MUST be able to accept the GSS_C_NT_ANONYMOUS name form
   and the GSS_KRB5_NT_PRINCIPAL_NAME name forms, and consider them

   Portable initiators are RECOMMENDED to use default credentials
   whenever possible, and request anonymity only through the input
   anon_req_flag to GSS_Init_Sec_Context().

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6.  Security Considerations

   Since KDCs ignore unknown options [RFC4120], a client requiring
   anonymous communication needs to make sure that the ticket is
   actually anonymous.  A KDC that that does not understand the
   anonymous option would not return an anonymous ticket.

   By using the mechanism defined in this specification, the client does
   not reveal its identity to the server but its identity may be
   revealed to the KDC of the server principal (when the server
   principal is in a different realm than that of the client), and any
   KDC on the cross-realm authentication path.  The Kerberos client MUST
   verify the ticket being used are indeed anonymous before
   communicating with the cross-realm KDC or the server, otherwise the
   client's identity may be revealed to the server unintentionally.

   In cases where specific server principals must not have access to the
   client's identity (for example, an anonymous poll service), the KDC
   can define server principal specific policy that insure any normal
   service ticket can NEVER be issued to any of these server principals.

7.  Acknowledgements

   The authors would like to thank the following individuals for their
   insightful comments and fruitful discussions: Sam Hartman, Martin
   Rex, Nicolas Williams, Jeffery Altman, Tom Yu, Chaskiel M Grundman,
   Love Hoernquist Aestrand, Jeffery Hutzelman, and Clifford Neuman.

8.  IANA Considerations

   No IANA actions are required for this document.

9.  Normative References

   [KRBNAM]   Zhu, L., "Additonal Kerberos Naming Contraints",
              draft-ietf-krb-wg-naming, work in progress.

   [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
              RFC 1964, June 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC2743]  Linn, J., "Generic Security Service Application Program

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              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

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Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


   Paul Leach
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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