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Network Working Group                                        M. Horowitz
<draft-ietf-cat-kerb-chg-password-02.txt>                Stonecast, Inc.
Internet-Draft                                              August, 1998

                   Kerberos Change Password Protocol

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   Distribution of this memo is unlimited.  Please send comments to the
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Abstract

   The Kerberos V5 protocol [RFC1510] does not describe any mechanism
   for users to change their own passwords.  In order to promote
   interoperability between workstations, personal computers, terminal
   servers, routers, and KDC's from multiple vendors, a common password
   changing protocol is required.



Overview

   When a user wishes to change his own password, or is required to by
   local policy, a simple request of a password changing service is
   necessary.  This service must be implemented on at least one host for
   each Kerberos realm, probably on one of the kdc's for that realm.
   The service must accept requests on UDP port 464 (kpasswd), and may
   accept requests on TCP port 464 as well.

   The protocol itself consists of a single request message followed by
   a single reply message.  For UDP transport, each message must be
   fully contained in a single UDP packet.








Horowitz                                                        [Page 1]


Internet Draft      Kerberos Change Password Protocol       August, 1998


Request Message

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         message length        |    protocol version number    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AP_REQ length        |         AP-REQ data           /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                        KRB-PRIV message                       /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   message length (16 bits)
      Contains the length of the message, including this field, in bytes
      (big-endian integer)
   protocol version number (16 bits)
      Contains the hex constant 0x0001 (big-endian integer)
   AP-REQ length (16 bits)
      length (big-endian integer) of AP-REQ data, in bytes.
   AP-REQ data, as described in RFC1510 (variable length)
      This AP-REQ must be for the service principal
      kadmin/changepw@REALM, where REALM is the REALM of the user who
      wishes to change his password.  The Ticket in the AP-REQ must be
      derived from an AS request (thus having the INITIAL flag set), and
      must include a subkey in the Authenticator.
   KRB-PRIV message, as described in RFC1510 (variable length)
      This KRB-PRIV message must be generated using the subkey in the
      Authenticator in the AP-REQ data.  The user-data component of the
      message must consist of the user's new password.

   The server must verify the AP-REQ message, decrypt the new password,
   perform any local policy checks (such as password quality, history,
   authorization, etc.) required, then set the password to the new value
   specified.

   The principal whose password is to be changed is the principal which
   authenticated to the password changing service.  This protocol does
   not address administrators who want to change passwords of principal
   besides their own.


Reply Message

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         message length        |    protocol version number    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AP_REP length        |         AP-REP data           /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                KRB-PRIV or KRB-ERROR message                  /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   message length (16 bits)



Horowitz                                                        [Page 2]


Internet Draft      Kerberos Change Password Protocol       August, 1998


      Contains the length of the message, including this field, in bytes
      (big-endian integer),
   protocol version number (16 bits)
      Contains the hex constant 0x0001 (big-endian integer)
   AP-REP length (16 bits)
      length of AP-REP data, in bytes.  If the the length is zero, then
      the last field will contain a KRB-ERROR message instead of a KRB-
      PRIV message.
   AP-REP data, as described in RFC1510 (variable length)
      The AP-REP corresponding to the AP-REQ in the request packet.
   KRB-PRIV or KRB-ERROR message, as described in RFC1510 (variable
      length)
      If the AP-REP length is zero, then this field contains a KRB-ERROR
      message.  Otherwise, it contains a KRB-PRIV message.  This KRB-
      PRIV message must be generated using the subkey in the
      Authenticator in the AP-REQ data.

      The user-data component of the KRB-PRIV message, or e-data
      component of the KRB-ERROR message, must consist of the following
      data:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          result code          |        result string          /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      result code (16 bits)
         The result code must have one of the following values (big-
         endian integer):
         0x0000 if the request succeeds.  (This value is not permitted
            in a KRB-ERROR message.)
         0x0001 if the request fails due to being malformed
         0x0002 if the request fails due to a "hard" error processing
            the request (for example, there is a resource or other
            problem causing the request to fail)
         0x0003 if the request fails due to an error in authentication
            processing
         0x0004 if the request fails due to a "soft" error processing
            the request (for example, some policy or other similar
            consideration is causing the request to be rejected).
         0xFFFF if the request fails for some other reason.
         Although only a few non-zero result codes are specified here,
         the client should accept any non-zero result code as indicating
         failure.
      result string (variable length)
         This field should contain information which the server thinks
         might be useful to the user, such as feedback about policy
         failures.  The string must be encoded in UTF-8.  It may be
         omitted if the server does not wish to include it.  If it is
         present, the client should display the string to the user.
         This field is analogous to the string which follows the numeric
         code in SMTP, FTP, and similar protocols.




Horowitz                                                        [Page 3]


Internet Draft      Kerberos Change Password Protocol       August, 1998


Dropped and Modified Messages

   An attacker (or simply a lossy network) could cause either the
   request or reply to be dropped, or modified by substituting a KRB-
   ERROR message in the reply.

   If a request is dropped, no modification of the password/key database
   will take place.  If a reply is dropped, the server will (assuming a
   valid request) make the password change.  However, the client cannot
   distinguish between these two cases.

   In this situation, the client should construct a new authenticator,
   re-encrypt the request, and retransmit.  If the original request was
   lost, the server will treat this as a valid request, and the password
   will be changed normally.  If the reply was lost, then the server
   should take care to notice that the request was a duplicate of the
   prior request, because the "new" password is the current password,
   and the password change time is within some implementation-defined
   replay time window.  The server should then return a success reply
   (an AP-REP message with result code == 0x0000) without actually
   changing the password or any other information (such as modification
   timestamps).

   If a success reply was replaced with an error reply, then the
   application performing the request would return an error to the user.
   In this state, the user's password has been changed, but the user
   believes that it has not.  If the user attempts to change the
   password again, this will probably fail, because the user cannot
   successfully provide the old password to get an INITIAL ticket to
   make the request.  This situation requires administrative
   intervention as if a password was lost.  This situation is,
   unfortunately, impossible to prevent.


Security Considerations

   This document deals with changing passwords for Kerberos.  Because
   Kerberos is used for authentication and key distribution, it is
   important that this protocol use the highest level of security
   services available to a particular installation.  Mutual
   authentication is performed, so that the server knows the request is
   valid, and the client knows that the request has been received and
   processed by the server.

   There are also security issues relating to dropped or modified
   messages which are addressed explicitly.


References

   [RFC1510] Kohl, J. and Neuman, C., "The Kerberos Network
      Authentication Service (V5)", RFC 1510, September 1993.





Horowitz                                                        [Page 4]


Internet Draft      Kerberos Change Password Protocol       August, 1998


Author's Address

   Marc Horowitz
   Stonecast, Inc.
   108 Stow Road
   Harvard, MA 01451

   Phone: +1 978 456 9103
   Email: marc@stonecast.net
















































Horowitz                                                        [Page 5]


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