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Network Working Group                                  D. Borman, EditorRequest for Comments: 1411                           Cray Research, Inc.                                                            January 1993Telnet Authentication: Kerberos Version 4Status 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     42.  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 hisTelnet 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 SETelnet Working Group                                            [Page 3]

RFC 1411             Kerberos Version 4 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   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.COMChair'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.comTelnet Working Group                                            [Page 4]