Network Working Group                                       K. Alagappan
Request for Comments: 1412                 Digital Equipment Corporation
                                                            January 1993


                       Telnet Authentication: SPX

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  Discussion and suggestions for improvement are requested.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

1. Command Names and Codes

   Authentication Types

      SPX          3

   Suboption Commands

      AUTH         0
      REJECT       1
      ACCEPT       2

2.  Command Meanings

   IAC SB AUTHENTICATION IS  AUTH
    IAC SE

      This is used to pass the SPX authentication token to the remote
      side of the connection.  (A document which describes the
      authentication token syntax is forthcoming.)  The first octet of
      the  value is SPX.  The second octet is
      a modifier to the SPX authentication type.

   IAC SB AUTHENTICATION REPLY  ACCEPT
    IAC SE

      This command indicates that the authentication was successful.
      After an SPX authentication exchange, both sides have securely
      established a random 8-byte key to be used as the default key for
      the ENCRYPTION option.  If the AUTH_HOW_MUTUAL bit is set in the
      second octet of the authentication-type-pair, the sender includes
      the mutual response bytes.  The receiver of the ACCEPT command
      compares the "mutual response" with its expected mutual response.



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RFC 1412                     SPX for Telnet                 January 1993


      (A document which describes the mutual response syntax is forth
      coming.)  If the AUTH_HOW_ONE_WAY bit is set in the second octet
      of the authentication-type-pair, the sender includes zero bytes of
      mutual response.

   IAC SB AUTHENTICATION REPLY  REJECT
    IAC SE

      This command indicates that the authentication was not successful,
      and if there is any more data in the sub-option, it is an ASCII
      text message of the reason for the rejection.

3.  Implementation Rules

   Every command after the first AUTHENTICATION IS must carry the same
   set of modifiers (e.g., CLIENT|MUTUAL) for subsequent AUTHENTICATION
   IS and AUTHENTICATION REPLY commands.

   If the second octet of the authentication-type-pair has the AUTH_WHO
   bit set to AUTH_WHO_CLIENT, then the client sends the initial AUTH
   command, and the server responds with either ACCEPT or REJECT.

   If the second octet of the authentication-type-pair has the AUTH_WHO
   bit set to AUTH_WHO_SERVER, then the server sends the initial AUTH
   command, and the client responds with either ACCEPT or REJECT.

4.  Examples

   User "joe" may wish to log in as user "pete" on machine "foo".  If
   "pete" has set things up on "foo" to allow "joe" access to his
   account, then the client would send IAC SB AUTHENTICATION NAME "pete"
   IAC SE IAC SB AUTHENTICATION IS SPX AUTH  IAC SE.  The server would then authenticate the user as "joe"
   from the token information, and the server would send back either
   ACCEPT or REJECT.  If mutual authentication is being used, the server
   would include in the ACCEPT message, a mutual response.  The
   authorization check to see if "pete" is allowing "joe" to use his
   account is made after the authentication exchange is complete.
   Therefore, it is possible for the client to receive an ACCEPT
   response (based on the authentication token), but for joe to be
   denied access to log in to pete's account.










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RFC 1412                     SPX for Telnet                 January 1993


       Client                           Server
                                        IAC DO AUTHENTICATION
       IAC WILL AUTHENTICATION

       [ The server is now free to request authentication information.
         ]

                                        IAC SB AUTHENTICATION SEND SPX
                                        CLIENT|MUTUAL SPX CLIENT|ONE_WAY
                                        IAC SE

       [ The server has requested mutual SPX authentication.  If mutual
         authentication is not supported, then the server is willing to
         do one-way SPX authentication.  ]

       [ The client will now respond with the name of the user that it
         wants to log in as, and the SPX authentication token.  ]

       IAC SB AUTHENTICATION NAME
       "pete" IAC SE
       IAC SB AUTHENTICATION IS SPX
       CLIENT|MUTUAL AUTH  IAC SE

       [ The server responds with an ACCEPT command to state that the
         authentication was successful.  ]

       [ If AUTH_HOW_MUTUAL, the server responds with the mutual
         response so the client can verify that it is really talking to
         the right server.  ]

       [ If AUTH_HOW_ONE_WAY, the server responds with a NULL mutual
         response, since the client is willing to trust the server
         already.  ]

                                        IAC SB AUTHENTICATION REPLY SPX
                                        CLIENT|MUTUAL ACCEPT  IAC SE












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RFC 1412                     SPX for Telnet                 January 1993


Security Considerations

   The ability to negotiate a common authentication mechanism between
   client and server is a feature of the authentication option that
   should be used with caution.  When the negotiation is performed, no
   authentication has yet occurred.  Therefore, each system has no way
   of knowing whether or not it is talking to the system it intends.  An
   intruder could attempt to negotiate the use of an authentication
   system which is either weak, or already compromised by the intruder.

Author's Address

   Kannan Alagappan
   Digital Equipment Corporation
   550 King Street, LKG1-2/A19
   Littleton, MA 01460

   EMail: kannan@sejour.lkg.dec.com

   Mailing List: telnet-ietf@CRAY.COM


   The working group can be contacted via the current chair:

   Steve Alexander
   INTERACTIVE Systems Corporation
   1901 North Naper Boulevard
   Naperville, IL 60563-8895

   Phone: (708) 505-9100 x256
   EMail: stevea@isc.com




















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