Kerberos Working Group                                         M. Swift
Internet Draft                                         University of WA
Document: draft-swift-win2k-krb-user2user-03.txt              J. Brezak
Category: Informational                                       Microsoft
                                                               P. Moore
                                                    Sandia National Labs
                                                           October 2001

           User to User Kerberos Authentication using GSS-API

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
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1. Abstract

   This draft documents a simple extension to the Kerberos GSS-API
   mechanism to support user to user authentication both in the case
   where the client application explicitly requests user to user
   authentication and when it does not know whether the server supports
   user to user authentication.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC-2119 [2].

3. Introduction

   The Kerberos user to user authentication mechanism allows for a
   client application to connect to a service that is not in possession
   of a long term secret key. Instead, the session ticket from the
   KERB-AP-REQ is encrypted using the session key from the service's
   ticket granting ticket. According to RFC 1510 [3]:

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        If the ENC-TKT-IN-SKEY option has been specified and an
        additional ticket has been included in the request, the KDC
        will decrypt the additional ticket using the key for the server
        to which the additional ticket was issued and verify that it is
        a ticket-granting ticket. If the request succeeds, the session
        key from the additional ticket will be used to encrypt the new
        ticket that is issued instead of using the key of the server
        for which the new ticket will be used (This allows easy
        implementation of user-to-user authentication, which uses
        ticket-granting ticket session keys in lieu of secret server
        keys in situations where such secret keys could be easily

   RFC2078 [5], in section 5.2, discusses a "Double-TGT K-5" mechanism
   and scenario, but not in the detail required in order to implement
   the mechanism.  The RFC for the Kerberos V5 GSS-API mechanism at the
   time this draft was prepared, RFC 1964 [4] does not support user-to-
   user authentication.

   This draft provides details as to mechanism type, token identifiers,
   messages and message types sufficient to document an implementation
   of user-to-user authentication in Kerberos GSS-API.  It follows the
   scenario described in RFC2078.

   The approach documented in this draft has been used to support user-
   to-user authentication in the Microsoft Windows 2000 SSPI with the
   Kerberos V5 protocol, and in a patched Kerberos V5 implementation
   being used to support a computing grid at Sandia, Lawrence
   Livermore, and Los Alamos National Laboratories.

4. User to User as a New Mechanism

   A new mechanism OID may be used to establish a user-to-user session:

        {iso(1) member-body(2) United States(840) mit(113554)
        infosys(1) gssapi(2) krb5(2) usertouser(3)}

   In the case that the client application knows that the server
   requires user-to-user authentication, then the initial call to
   GSS_Init_Sec_Context will request this mechanism. This new mechanism
   is used with a token exchange that extends the conventional Kerberos
   GSS-API protocol by adding an additional round trip to request the
   TGT from the service. As with all Kerberos GSS-API messages, the
   following tokens are encapsulated in the GSS-API framing. The first
   token of the exchange will have an innerContextToken with a 2-octet
   TOK_ID field containing 04 00 (KERB-TGT-REQUEST) followed by a
   Kerberos V5 message as follows:

                pvno[0]                         INTEGER,
                msg-type[1]                     INTEGER,
                server-name[2]                  PrincipalName OPTIONAL,
                realm[3]                        Realm OPTIONAL

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   The TGT request consists of four fields:

        pvno and msg-type are as defined in RFC1510 section 5.4.1. msg-
                type is KRB_TGT_REQ (16).

        server-name : this field optionally contains the name of the
                server. If the client application doesn't know the
                server name this can be left blank and the server
                application will pick the appropriate server
                credentials which may be the default credential.

        realm : this field optionally contains the realm of the server.
                If the client application doesn't know the server realm
                this field can be left blank and the server application
                will pick the appropriate server credentials which may
                be the server's default realm.

   The server name and realm are included to allow a server application
   to act for multiple principles in different realms and to choose
   which credentials to use.

   The response to the KERB-TGT-REQUEST message will be a
   KERB_TGT_REPLY token which will have an innerContextToken with a 2-
   octet TOK_ID field containing 04 01 (KERB-TGT-REPLY) followed by a
   Kerberos V5 message as follows:

                pvno[0]                         INTEGER,
                msg-type[1]                     INTEGER,
                ticket[2]                       Ticket

   The TGT reply contains the following fields:

        pvno and msg-type are as defined in RFC1510 section 5.4.1. msg-
                type is KRB_TGT_REP (17)

        ticket : contains the TGT for the service specified by the
                server name and realm passed by the client or the
                default service.

   If the service does not possess a ticket granting ticket, it should
   return the error KRB_AP_ERR_NO_TGT (0x43).

   If the server name and realm in the KERB-TGT-REQUEST message do not
   match the name of the service, then the service should return the
   error KRB_AP_ERR_NOT_US.

   Following the exchange of the TGT request messages, the initiator
   requests a ticket to the service from the KDC using a KERB-TGS-REQ
   with the KDCoption ENC-TKT-IN_SKEY and the second ticket in the

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   additional-tickets of the KDC-REQ-BODY. Upon receipt of the TGS-REP
   the rest of the authentication identical to the Kerberos GSS-API
   mechanism defined in RFC 1964 [4].

5. User-to-User when applied via KDC policy

   Implementations MAY support the ability apply a policy on a user
   account such that the KDC will not allow conventional service ticket
   requests, and when presented with a KERB_TGS_REQ that does not
   contain a second ticket with an ENC_TKT_IN_SKEY KDCoption will
   respond with a KRB-ERROR with the msg-type

   In this case, the client need not explicitly request user-to-user in
   order to get a user-to-user connection. Implementations may use this
   error code to set a flag and return a GSS_C_CONTINUE_NEEDED so that
   the next round uses the mechanism described in section 4.

6. User to User when applied by server policy

   In the case that the client application doesn't know that a service
   requires user-to-user authentication, and requests and receives a
   conventional KRB_AP_REP, the client will send the KRB_AP_REP
   request, and the server will respond with a KRB_ERROR token as
   described in RFC1964, with a msg-type of
   KRB_AP_ERR_USER_TO_USER_REQUIRED (0x45). The server may optionally
   pass the TGT in the data field of this error message. In response to
   this error, the initiator sets flags and returns a
   GSS_C_CONTINUE_NEEDED so that the next round uses the mechanism
   described in section 4.

7. Security Considerations

   These extensions simply enable an existing Kerberos 5 authentication
   protocol so that it may be used from GSSAPI.

   There is some risk in a server handing out its ticket-granting-
   ticket to any client that requests it, in that it gives an attacker
   a piece of encrypted material to decrypt. However, the same material
   may be obtained from listening to any legitimate client connection.

   It should be noted here that the exchange described in section 6
   requires that the KDC provide tickets for user accounts, which will
   contain known plaintext encrypted in the usersÆ private key. The
   risk associated with this is entirely mitigated where a KDC supports
   the KDC_MUST_USE_USER2USER feature, which allows a restriction on
   user accounts to ensure that all tickets for that account are
   encrypted in the TGT session key, and not the long term key of the

8. References

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   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

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

   3  J. Kohl, C. Neuman, "The Kerberos Network Authentication
      Service(V5)",  RFC 1510.

   4  J. Linn, "The Kerberos Version 5 GSS-API Mechanism", RFC 1964

   5  J. Linn, "Generic Security Service Application Program Interface,
      Version 2", RFC 2078

9. Author's Addresses

   Michael Swift
   University of Washington
   Seattle, Washington

   John Brezak
   One Microsoft Way
   Redmond, Washington

   Patrick Moore
   Sandia National Laboratories
   PO Box 5800 Mail Stop
   Albuquerque, New Mexico

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