NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Updates: 4120 (if approved)                                K. Jaganathan
Expires: January 17, 2007                          Microsoft Corporation
                                                           July 16, 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 . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   9.  Normative References . . . . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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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

   The anonymous Kerberos realm name is a reserved realm name as defined
   in [KRBNAM] and its value is the literal "RESERVED:ANONYMOUS".

   The anonymous Kerberos principal name is a reserved Kerberos
   principal name as defined in [KRBNAM], its name-type [RFC4120] is
   KRB_NT_RESRVED [KRBNAM], and its name-string [RFC4120] is a sequence
   of two KerberosString components: "RESERVED", "ANONYMOUS".

   In this specification, only the client name or the client realm can
   be anonymous; the server name or the server realm can not be

   The transited field [RFC4120] of a ticket is an anonymous
   authentication path if the tr-type field of the TransitedEncoding

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   type [RFC4120] is NO-TRANSITED-INFO and the contents field is an
   empty OCTET STRING.


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

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

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

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

   o  The cname field [RFC4120] contains the anonymous Kerberos
      principal name.

   o  The crealm field [RFC4120] contains either the realm name of the
      client who made the request or the anonymous kerberos realm name,
      based on the local policy of the KDC.

   o  The transited field [RFC4120] can contain either the client's
      "normal" authentication path according to Section of
      [RFC4120] or the anonymous authentication path.

   o  It contains no information that can reveal the client's identity.
      However the ticket can contain the client realm and the realms on
      the authentication path, and the authorization data may provide
      additional information of the client.  For example, an anonymous
      principal that is only identifiable within a particular group of
      users can be implemented by using authorization data.

   o  The anonymous ticket flag is set.

   Notes: The anonymous ticket flag MUST NOT be set by implementations
   of this specification if the ticket is not an anonymous ticket.  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.

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

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

4.  Protocol Description

   In order to request an anonymous ticket, the client sets the request-
   anonymous KDC option in an Authentication Exchange (AS) or Ticket
   Granting Service (TGS) request [RFC4120].  The client can request an
   anonymous TGT based on a normal TGT.  Note that if the ticket in the
   PA-TGS-REQ [RFC4120] is anonymous, the request-anonymous KDC option
   MUST be set in the request.

   When propagating authorization data, care MUST be taken by the TGS to
   ensure that the client confidentiality is not violated: the TGS MUST
   either fail the request or remove authorization data that may reveal
   the client's identity.  An optional authorization element unknown by
   the TGS MUST be removed if it can be ignored (such as ones enclosed
   in the AD-IF-RELEVANT or the AD-KDCIssued containers [RFC4120]).  The
   TGS can strip critical unknown authorization data if such data do not
   convey any rights based on the requesting client's identity.  Here is
   a table of the known authorization-data elements, flagged with
   whether they interfere with client anonymity and recommendations for
   how to process them.

         ad-type          References  Can Breach Confidentiality?
     AD-IF-RELEVANT        RFC4120     Yes, remove if unknown
     AD-KDCIssued          RFC4120     Yes, remove if unknown
     AD-AND-OR             RFC4120     Yes, remove if unknown
     AD-MANDATORY-FOR-KDC  RFC4120     Yes, fail the request if unknown

   If it is inappropriate to remove an authorization element 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].

   When policy allows, the KDC issues an anonymous ticket.  The client
   realm in the anonymous ticket can be the anonymous realm name based
   on local policy.  The client name and the client realm the
   EncKDCRepPart of the reply [RFC4120] MUST match with the
   corresponding client name and the client realm of the anonymous reply
   ticket.  The client then MUST use the client name and the client
   realm returned in the EncKDCRepPart in subsequent message exchanges
   when using that anonymous ticket.

   If there is a key known by both the client and the KDC for encrypting
   the KDC reply, the cname field in the request [RFC4120] can be

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   anonymous.  If the client is anonymous and the KDC does not have a
   key to encrypt the reply, the KDC MUST return an error message with
   the code KDC_ERR_NULL_KEY [RFC4120].  For AS exchange, if the reply
   key is selected from the client keys (for example, as described in
   Section 3.1.3 of [RFC4120]), then the client principal MUST NOT be
   anonymous.  The client can use the client keys to request an
   anonymous TGT in the AS request.  The anonymous client name, for
   example, can be used in conjunction with PKINIT [RFC4556].  An
   anonymous PKINIT client can authenticate the KDC based on the KDC
   certificate.  For TGS exchange, the reply key is selected according
   to Section 3.3.3 of [RFC4120] as normal.

   The KDC fills out the transited field of the anonymous ticket in the
   reply as follows: If the service ticket in a TGS request is an
   anonymous ticket with a "normal" authentication path, then the
   authentication path in the reply ticket MUST also contain a "normal"
   authentication path: the TGS MUST add the name of the previous realm.
   However, if the service ticket in a TGS request is an anonymous
   ticket with an anonymous authentication path, then the reply ticket
   can contain either an anonymous authentication path or a "normal"
   authentication path, based on the local policy of the KDC.  Thus a
   "normal" authentication path in an anonymous ticket can be a partial
   path: it may not include all the intermediate realms on the
   authentication path.

   The KDC fills out the authtime field of the anonymous ticket in the
   reply as follows: If the anonymous ticket is returned in an AS
   exchange, the authtime field of the ticket contains the request time.
   If the anonymous ticket is returned in a TGS exchange, the authtime
   field contains the time of the initial authentication for the
   principal who has made the request.  An anonymous ticket can be
   renewed, and the authtime field of a renewed ticket is the authtime
   in the anonymous ticket that the renewed ticket was based on.

   If a client requires anonymous communication then the client MUST
   check to make sure that the ticket in the reply 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].  No transited policy checking is needed for
   the anonymous authentication path.  However, transited policy checks
   defined in Section 2.7 of [RFC4120] would apply to an anonymous
   ticket that contains a "normal" authentication path.

   A server accepting an anonymous service ticket may assume that

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   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 the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
   represent the anonymous identity.  In addition, Section 2.1.1 of
   [RFC1964] defines the single string representation of a Kerberos
   principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME.  For
   the anonymous principals, the name component within the exportable
   name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm
   name according to Section 2.1.1 of [RFC1964].  In this specification
   only the client/initiator can be the anonymous identity.

   Portable initiators are RECOMMENDED to use default credentials
   whenever possible, and request anonymity only through the input
   anon_req_flag [RFC2743] 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 is 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.

   If the KDC that issued an anonymous ticket were to maintain records
   of the association of identities to an anonymous ticket, then someone
   obtaining such records could breach the anonymity.  Additionally, the
   implementation of most (for now all) KDC's respond to requests at the
   time that they are received.  Traffic analasys on the connection to
   the KDC will allow an attacket to match client identities to
   anonymous tickets issued.  Because there are plaintext parts of the
   tickets that are exposed on the wire, such matching by a third party
   observer is relatively straigtforward.

7.  Acknowledgements

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

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.

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   [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
              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.

   [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
              Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.

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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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