Network Working Group                                  D. Borman, Editor
Request for Comments: 1411                           Cray Research, Inc.
                                                            January 1993


               Telnet Authentication: Kerberos Version 4

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

      KERBEROS_V4  1

   Suboption Commands

      AUTH         0
      REJECT       1
      ACCEPT       2
      CHALLENGE    3
      RESPONSE     4

2.  Command Meanings

   IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH <kerberos
   ticket and authenticator> IAC SE

      This is used to pass the Kerberos ticket to the remote side of the
      connection.  The first octet of the <authentication-type-pair>
      value is KERBEROS_V4, to indicate the usage of Kerberos version 4.

   IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT IAC SE

      This command indicates that the authentication was successful.

   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.

   IAC SB AUTHENTICATION IS <authentication-type-pair> CHALLENGE
   <encrypted challenge> IAC SE
   IAC SB AUTHENTICATION REPLY <authentication-type-pair> RESPONSE
   <encrypted response> IAC SE

      These two commands are used to perform mutual authentication.
      They are only used when the AUTH_HOW_MUTUAL bit is set in the
      second octet of the authentication-type-pair.  After successfully
      sending an AUTH and receiving an ACCEPT, a CHALLENGE is sent.  The
      challenge is a random 8 byte number with the most significant byte
      first, and the least significant byte last.  When the CHALLENGE
      command is sent, the "encrypted challenge" is the 8-byte-challenge
      encrypted in the session key.  When the CHALLENGE command is
      received, the contents are decrypted to get the original 8-byte-
      challenge, this value is then incremented by one, re-encrypted
      with the session key, and returned as the "encrypted response" in
      the RESPONSE command.  The receiver of the RESPONSE command
      decrypts the "encrypted response", and verifies that the resultant
      value is the original 8-byte-challenge incremented by one.

      The "encrypted challenge" value sent/received in the CHALLENGE
      command is also encrypted with the session key on both sides of
      the session, to produce a random 8-byte key to be used as the
      default key for the ENCRYPTION option.

3.  Implementation Rules

   If the second octet of the authentication-type-pair has the AUTH_WHO
   bit set to AUTH_CLIENT_TO_SERVER, then the client sends the initial
   AUTH command, and the server responds with either ACCEPT or REJECT.
   In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, and the
   server responds with ACCEPT, then the client then sends a CHALLENGE,
   and the server sends a RESPONSE.

   If the second octet of the authentication-type-pair has the AUTH_WHO
   bit set to AUTH_SERVER_TO_CLIENT, then the server sends the initial
   AUTH command, and the client responds with either ACCEPT or REJECT.
   In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, and the
   client responds with ACCEPT, then the server then sends a CHALLENGE,
   and the client sends a RESPONSE.

   The authenticator (Kerberos Principal) used is of the form
   "rcmd.host@realm".

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 KERBEROS_V4 AUTH <joe's kerberos
   ticket> IAC SE The server would then authenticate the user as "joe"
   from the ticket information, and since "pete" is allowing "joe" to
   use his account, the server would send back ACCEPT.  If mutual
   authentication is being used, the the client would send a CHALLENGE,
   and verify the RESPONSE that the server sends back.

       Client                           Server
                                        IAC DO AUTHENTICATION
       IAC WILL AUTHENTICATION
       [ The server is now free to request authentication information.
         ]
                                        IAC SB AUTHENTICATION SEND
                                        KERBEROS_V4 CLIENT|MUTUAL
                                        KERBEROS_V4 CLIENT|ONE_WAY IAC
                                        SE
       [ The server has requested mutual Version 4 Kerberos
         authentication.  If mutual authentication is not supported,
         then the server is willing to do one-way authentication.

         The client will now respond with the name of the user that it
         wants to log in as, and the Kerberos ticket.  ]
       IAC SB AUTHENTICATION NAME
       "pete" IAC SE
       IAC SB AUTHENTICATION IS
       KERBEROS_V4 CLIENT|MUTUAL AUTH
       <kerberos ticket information>
       IAC SE
       [ The server responds with an ACCEPT command to state that the
         authentication was successful.  ]
                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V4 CLIENT|MUTUAL ACCEPT
                                        IAC SE
       [ Next, the client sends across a CHALLENGE to verify that it is
         really talking to the right server.  ]
       IAC SB AUTHENTICATION IS
       KERBEROS_V4 CLIENT|MUTUAL
       CHALLENGE xx xx xx xx xx xx xx
       xx IAC SE
       [ Lastly, the server sends across a RESPONSE to prove that it
         really is the right server.
                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V4 CLIENT|MUTUAL
                                        RESPONSE yy yy yy yy yy yy yy yy
                                        IAC SE

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

   David A. Borman, Editor
   Cray Research, Inc.
   655F Lone Oak Drive
   Eagan, MN 55123

   Phone: (612) 452-6650
   EMail: dab@CRAY.COM

   Mailing List: telnet-ietf@CRAY.COM

Chair's Address

   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