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Network Working Group                                       K. AlagappanRequest for Comments: 1412                 Digital Equipment Corporation                                                            January 1993Telnet Authentication: SPXStatus 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       22.  Command Meanings   IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH   <SPX authentication token> 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 <authentication-type-pair> value is SPX.  The second octet is      a modifier to the SPX authentication type.   IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT   <mutual response> 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.Telnet Working Group                                            [Page 1]

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 <authentication-type-pair> REJECT   <optional reason for rejection> 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 <joe's spx authentication   token> 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.Telnet Working Group                                            [Page 2]

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 <spx       authentication token       information> 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 <mutual                                        response> IAC SETelnet Working Group                                            [Page 3]

RFC 1412                     SPX for Telnet                 January 1993Security 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.comTelnet Working Group                                            [Page 4]