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.



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RFC 1411             Kerberos Version 4 for Telnet          January 1993


  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



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RFC 1411             Kerberos Version 4 for Telnet          January 1993


  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





Telnet Working Group                                            [Page 3]

RFC 1411             Kerberos Version 4 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

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

  Phone: (612) 452-6650
  EMail: [email protected]

  Mailing List: [email protected]

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: [email protected]


















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