

Kerberos Working Group                                         M. Swift 
Internet Draft                                         University of WA 
Document: draft-swift-win2k-krb-user2user-02.txt              J. Brezak 
Category: Informational                                       Microsoft 
                                                               P. Moore 
                                                    Sandia National Labs
                                                              April 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 
   Task Force (IETF), its areas, and its working groups. Note that 
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of 
   six months and may be updated, replaced, or obsoleted by other 
   documents at any time. It is inappropriate to use Internet-Drafts as 
   reference material or to cite them other than as "work in progress."  
    
   The list of current Internet-Drafts can be accessed at 
   http://www.ietf.org/ietf/1id-abstracts.txt  
   The list of Internet-Draft Shadow Directories can be accessed at 
   http://www.ietf.org/shadow.html. 
    
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", 
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in 
   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 
        compromised). 
    
   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: 
    
        KERB-TGT-REQUEST ::= SEQUENCE { 
                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: 
    
        KERB-TGT-REPLY ::= SEQUENCE { 
                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 
   KDC_ERR_MUST_USE_USER2USER (or KRB5PLACEHOLD_27). 
    
   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 usersĘ 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 
   user. 
    
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 
   Email: mikesw@cs.washington.edu 
    
   John Brezak 
   Microsoft 
   One Microsoft Way 
   Redmond, Washington 
   Email: jbrezak@microsoft.com 
    
   Patrick Moore 
   Sandia National Laboratories 
   PO Box 5800 Mail Stop 
   Albuquerque, New Mexico 
   Email: pcmoore@sandia.gov




















  
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