draft-ietf-krb-wg-anon-02.txt   [plain text]

NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Updates: 4120 (if approved)                                K. Jaganathan
Intended status: Standards Track                   Microsoft Corporation
Expires: April 14, 2007                                 October 11, 2006

                     Anonymity Support for Kerberos

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document defines extensions to the Kerberos protocol for the
   Kerberos client to authenticate the Kerberos Key Distribution Center
   and the Kerberos server, without revealing the client's identity.
   These extensions can be used to secure communication between the
   anonymous client and 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 . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   9.  Normative References . . . . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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1.  Introduction

   In certain situations, 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.

   Extensions to [RFC4120] are specified in this document by which a
   client can authenticate the KDC and request an anonymous ticket.  The
   client can use the anonymous ticket to authenticate the server and
   protect subsequent client-server communications.  These extensions
   provide Kerberos with functional equivalence to Transport Layer
   Security (TLS) [RFC4346].

   By using the extensions defined in this specification, the client MAY
   reveal its identity in its initial request to its own KDC, but it can
   remain anonymous thereafter to KDCs on the cross-realm authentication
   path, and to the server with which it communicates.

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 based on
   [KRBNAM].  The value is the literal "RESERVED:ANONYMOUS".

   The anonymous Kerberos principal name is a reserved Kerberos
   principal name based on [KRBNAM].  The value of the name-type field
   is KRB_NT_RESRVED [KRBNAM], and the value of the name-string field is
   a sequence of two KerberosString components: "RESERVED", "ANONYMOUS".

   Note that in this specification, the anonymous principal name and
   realm are only applicable to the client in Kerberos messages, the
   server MUST NOT be anonymous in any Kerberos message.

   The transited field [RFC4120] of a ticket is an anonymous
   authentication path if the tr-type field of the TransitedEncoding
   type [RFC4120] is NO-TRANSITED-INFO and the contents field is an

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   empty OCTET STRING.


   This means that no information of the authentication path is

   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 client's realm name
      or the anonymous realm name.

   o  The transited field [RFC4120] can contain either the client's
      authentication path as described in Section of [RFC4120]
      or the anonymous authentication path.

   o  The anonymous ticket contains no information that can reveal the
      client's identity.  However the ticket MAY contain the client
      realm and the realms on the authentication path, and authorization
      data that MAY provide information related to the client's
      identity.  For example, an anonymous principal that is only
      identifiable within a particular group of users can be implemented
      using authorization data and such authorization data, if included
      in the anonymous ticket, shall disclose the client's membership of
      that group.

   o  The anonymous ticket flag is set.

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

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

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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.  If the client wishes to
   authenticate the KDC anonymously, it sets the client name as
   anonymous in the AS exchange and provides a PA_PK_AS_REQ pre-
   authentication data [RFC4556] where both the signerInfos field and
   the certificates field of the SignedData [RFC3852] of PA_PK_AS_REQ
   are empty.  Because the anonymous client does not have an associated
   asymmetric key pair, the client MUST use the Diffie-Hellman key
   agreement method by filling in the Diffie-Hellman domain parameters
   in the clientPublicValue [RFC4556].

   If the ticket in the PA-TGS-REQ [RFC4120] of the TGS request is
   anonymous, or if the client in the AS request is anonymous, the
   request-anonymous KDC option MUST be set in the request.

   Upon receiving the AS request with a PA_PK_AS_REQ from the anonymous
   client, the KDC skips the checks for the client's signature and the
   client's public key (such as the verification of the binding between
   the client's public key and the client name), but performs otherwise-
   applicable checks, and proceeds as normal according to [RFC4556].
   For example, the AS MUST check if the client's Diffie-Hellman domain
   parameters are acceptable.  The Diffie-Hellman key agreement method
   MUST be used and the reply key is derived according to Section of [RFC4556].  If the clientPublicValue is not present in the
   request, the KDC MUST return a KRB-ERROR [RFC4120] with the code
   accompanying e-data.  The client that made the anonymous request can
   authenticate the KDC based on the KDC's signature in the reply.  If
   the KDC does not have an asymmetric key pair, it MAY reply
   anonymously.  In which case, both the signerInfos field and the
   certificates field of the SignedData [RFC3852] of PA_PK_AS_REP in the
   reply are empty.  The server name in an anonymous reply contains the
   name of the TGS.  Upon receipt of an anonymous KDC reply, the client
   MUST reject the returned ticket if it can not authenticate the KDC

   The client can use its keys to mutually authenticate with the KDC,
   and request an anonymous TGT in the AS request.  And in that case,
   the reply key is selected as normal according to Section 3.1.3 of

   For the TGS exchange, the reply key is selected as normal according
   to Section 3.3.3 of [RFC4120].

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

   During the TGS 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
   structure).  The TGS can only strip critical unknown authorization
   data if the ticket does not convey any rights such as those conveyed
   by a KDCIssued authorization data element.  If a ticket contains a
   KDCIssued authorization data element, then no other authorization
   data elements may be removed if they could serve to limit the rights
   conveyed by the KDCIssued element.  Here is a table of the known
   authorization-data elements, tagged 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, fail the request if unknown
     AD-AND-OR             RFC4120     Yes, remove if unknown
     AD-MANDATORY-FOR-KDC  RFC4120     Yes, fail the request if unknown

   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 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.

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   If the anonymous ticket is returned in a TGS exchange, the authtime
   field contains the authtime of the ticket in the PA-TGS-REQ
   [RFC4120].  An anonymous ticket can be renewed, and the authtime
   field of a renewed ticket is the authtime in the anonymous ticket on
   which the renewed ticket was based.

   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].

   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] and there is no accompanying e-data.

   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.  This is
   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
   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 the initiator/client to assert the
   "anonymous" flag when calling GSS_Init_Sec_Context().

   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 initiator/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

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   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].  Note that in this
   specification only the client/initiator can be anonymous.

   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().

6.  Security Considerations

   Since KDCs ignore unknown options [RFC4120], a client requiring
   anonymous communication needs to make sure that the ticket is
   actually anonymous.  This is because 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 server, otherwise the client's identity may be revealed

   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.

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   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
   implementations of most (for now all) KDC's respond to requests at
   the time that they are received.  Traffic analysis on the connection
   to the KDC will allow an attacker 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 straightforward.

7.  Acknowledgements

   Clifford Neuman contributed the core notions of this document.

   Martin Rex wrote the text for GSS-API considerations.

   Nicolas Williams reviewed the GSS-API considerations section and
   suggested ideas for improvements.

   Sam Hartman and Nicolas Williams were great champions of this work.

   In addition, the following individuals made significant
   contributions: Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love
   Hoernquist Aestrand, and Jeffery Hutzelman.

8.  IANA Considerations

   Section 3 defines the anonymous Kerberos name and the anonymous
   Kerberos realm based on [KRBNAM].  The IANA registry for [KRBNAM]
   need to be updated to add references to 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.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC3852]  Housley, R., "Cryptographic Message Syntax (CMS)",

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              RFC 3852, July 2004.

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

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Email: lzhu@microsoft.com

   Paul Leach
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Email: paulle@microsoft.com

   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Email: karthikj@microsoft.com

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