NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Updates: 4120 (if approved)                        Microsoft Corporation
Intended status: Standards Track                           July 27, 2008
Expires: January 28, 2009

                     Anonymity Support for Kerberos

Status of this Memo

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   This document defines extensions to the Kerberos protocol for the
   Kerberos client to authenticate the Kerberos Key Distribution Center
   (KDC) and the Kerberos server, without revealing the client's
   identity to the server or to the KDC.  It updates RFC 4120.  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 . . . . . . . . . . . . . . . . . . . . .  4
   5.  GSS-API Implementation Notes . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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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 Kerberos are specified in this document by which a
   client can authenticate the Key Distribution Center (KDC) and request
   an anonymous ticket.  The client can use the anonymous ticket to
   authenticate the server and protect subsequent client-server

   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 defined as a well-known realm
   name based on [KRBNAM].  The value is the literal "WELLKNOWN:

   The anonymous Kerberos principal name is defined as a well-known
   Kerberos principal name based on [KRBNAM].  The value of the name-
   type field is KRB_NT_WELLKNOWN [KRBNAM], and the value of the name-
   string field is a sequence of two KerberosString components:

   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]

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   This is a new ticket flag that is used to indicate a ticket is an
   anonymous one.

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

   o  The cname field contains the anonymous Kerberos principal name.

   o  The crealm field contains the client's realm name, or the name of
      the realm that issued the initial ticket for the client principal
      (when the anonymous ticket is obtained using anonymous Public Key
      Cryptography for Initial Authentication in Kerberos (PKINIT) as
      defined in Section 4, or the anonymous realm name.

   o  The anonymous ticket contains no information that can reveal the
      client's identity.  However the ticket may contain the client
      realm, intermediate realms on the client's authentication path,
      and authorization data that may provide information related to the
      client's identity.  For example, an anonymous principal that is
      identifiable only 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 anonymous KDC option is defined as bit 14 (with the first bit
   being bit 0) in the KDCOptions:

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

   As described in Section 4, the anonymous KDC option is set to request
   an anonymous ticket.

4.  Protocol Description

   In order to request an anonymous ticket, the client sets the
   anonymous KDC option in an Authentication Service exchange (AS) or
   Ticket Granting Service (TGS) exchange.  The client can request an
   anonymous Ticket Granting Ticket (TGT) based on a normal TGT.  Unless
   otherwise specified, the client can obtain an anonymous ticket with
   the anonymous realm name only by requesting an anonymous ticket in an
   AS exchange with the client realm set as anonymous in the request.

   If the client wishes to authenticate to the KDC anonymously, it sets

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   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 the PA_PK_AS_REQ are empty.  Because the anonymous
   client does not have an associated asymmetric key pair, the client
   MUST choose the Diffie-Hellman key agreement method by filling in the
   Diffie-Hellman domain parameters in the clientPublicValue [RFC4556].
   This use of the anonymous client name in conjunction with PKINIT is
   referred to as anonymous PKINIT.  If anonymous PKINIT is used, the
   realm name in the returned anonymous ticket MUST be the anonymous

   If the ticket in the PA-TGS-REQ of the TGS request is anonymous, or
   if the client in the AS request is anonymous, the anonymous KDC
   option MUST be set in the request.  Otherwise, the KDC MUST return a
   KRB-ERROR message with the code KDC_ERR_BADOPTION.

   Upon receiving the AS request with a PA_PK_AS_REQ [RFC4556] from the
   anonymous client, the KDC processes the request according to Section
   3.1.2 of [RFC4120].  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
   [RFC4556].  If all goes well, an anonymous ticket is generated
   according to Section 3.1.3 of [RFC4120] and a PA_PK_AS_REP [RFC4556]
   pre-authentication data is included in the KDC reply according to
   [RFC4556].  If the KDC does not have an asymmetric key pair, it MAY
   reply anonymously or reject the authentication attempt.  If the KDC
   replies anonymously, 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 the anonymous KDC reply contains the name
   of the TGS.

   A KDC that supports anonymous PKINIT MUST indicate the support of
   PKINIT according to Section 3.4 of [RFC4556].

   Upon receipt of the KDC reply that contains an anonymous ticket and a
   PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then
   authenticate the KDC based on the KDC's signature in the
   PA_PK_AS_REP.  If the KDC's signature is missing in the KDC reply
   (the reply is anonymous), the client MUST reject the returned ticket
   if it cannot authenticate the KDC otherwise.

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   The client can use the client keys to mutually authenticate with the
   KDC, 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].

   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 EncTicketPart of the
   reply 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 KDC-REP in
   subsequent message exchanges when using the obtained anonymous

   When propagating authorization data in the ticket or in the enc-
   authorization-data field of the request, the TGS MUST ensure that the
   client confidentiality is not violated in the returned anonymous
   ticket.  The TGS MUST process the authorization data recursively
   according to Section 5.2.6 of [RFC4120] beyond the container levels
   such that all embedded authorization elements are interpreted.  AS or
   TGS SHOULD NOT populate identity-based authorization data into an
   anonymous ticket in that such authorization data typically reveals
   the client's identity.  The specification of a new authorization data
   type MUST specify the processing rules of the authorization data when
   an anonymous ticket is returned.  If there is no processing rule
   defined for an authorization data element or the authorization data
   element is unknown, the TGS MUST process it when an anonymous ticket
   is returned as follows:

   o  If the authorization data element may reveal the client's
      identity, it MUST be removed unless otherwise specified.

   o  If the authorization data element is intended to restrict the use
      of the ticket or limit the rights otherwise conveyed in the
      ticket, it cannot be removed in order to hide the client's
      identity.  In this case, the authentication attempt MUST be
      rejected, and the KDC MUST return an error message with the code
      KDC_ERR_POLICY.  Note this is applicable to both critical and
      optional authorization data.

   o  If the authorization data element is unknown, the TGS MAY remove
      it, or transfer it into the returned anonymous ticket, or reject
      the authentication attempt, based on local policy for that
      authorization data type unless otherwise specified.  If there is

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      no policy defined for a given unknown authorization data type, the
      authentication MUST be rejected.  The error code is KDC_ERR_POLICY
      when the authentication is rejected.

   The AD-INITIAL-VERIFIED-CAS authorization data as defined in
   [RFC4556] contains the issuer name of the client certificate.  If it
   is undesirable to disclose such information about the client's
   identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be
   removed from an anonymous ticket based on local policy of the TGS.

   The TGS encodes the name of the previous realm into the transited
   field according to Section of [RFC4120].  Based on local
   policy, the TGS MAY omit the previous realm if the cross realm TGT is
   an anonymous one to hide the authentication path of the client.  The
   unordered set of realms in the transited field, if present, can
   reveal which realm may potentially be the realm of the client or the
   realm that issued the anonymous TGT.  The anonymous Kerberos realm
   name MUST NOT be present in the transited field of a ticket.  The
   true name of the realm that issued the anonymous ticket MAY be
   present in the transited field of a ticket.

   If the client is anonymous and the KDC does not have a key to encrypt
   the reply (this can happen when, for example, the KDC does not
   support PKINIT [RFC4556]), the KDC MUST return an error message with
   the code KDC_ERR_NULL_KEY [RFC4120].

   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 in the flags
   field of the EncKDCRepPart.  This is because KDCs ignore unknown KDC
   options.  A KDC that does not understand the 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].

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

   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.

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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 initiator name that is returned from the acceptor side
   upon the 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, a client requiring anonymous
   communication needs to make sure that the returned 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

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   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.

   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.

   The client's real identity is not revealed when the client is
   authenticated as the anonymous principal.  Application servers MAY
   reject the authentication in order to, for example, prevent
   information disclosure or as part of Denial of Service (DOS)
   prevention.  Application servers MUST avoid accepting anonymous
   credentials in situations where they must record the client's
   identity; for example, when there must be an audit trail.

7.  Acknowledgements

   JK Jaganathan helped editing early revisions of this document.

   Clifford Neuman contributed the core notions of this document.

   Ken Raeburn reviewed the document and provided suggestions for

   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.

   Miguel Garcia and Phillip Hallam-Baker reviewed the document and
   provided helpful suggestions.

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   In addition, the following individuals made significant
   contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love
   Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia.

8.  IANA Considerations

   This document defines a new 'anonymous' Kerberos well-known name and
   a new 'anonymous' Kerberos well-known realm based on [KRBNAM].  IANA
   is requested to add these two values to the Kerberos naming
   registries that are created in [KRBNAM].

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)",
              RFC 3852, July 2004.

   [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.

Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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   Paul Leach
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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