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RFC 1411

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


               Telnet Authentication: Kerberos Version 4

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.



Telnet Working Group                                            [Page 1]

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 Telnet Working Group [Page 2]
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.



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