Network Working Group                                  D. Borman, Editor
Request for Comments: 1409                           Cray Research, Inc.
                                                            January 1993


                      Telnet Authentication Option

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  37
       IS               0
       SEND             1
       REPLY            2
       NAME             3

       Authentication Types
       NULL             0
       KERBEROS_V4      1
       KERBEROS_V5      2
       SPX              3
       RSA              6
       LOKI            10

       Modifiers
       AUTH_WHO_MASK        1
       AUTH_CLIENT_TO_SERVER    0
       AUTH_SERVER_TO_CLIENT    1
       AUTH_HOW_MASK        2
       AUTH_HOW_ONE_WAY         0
       AUTH_HOW_MUTUAL          2

2.  Command Meanings

   This document makes reference to a "server" and a "client".  For the
   purposes of this document, the "server" is the side of the connection
   that did the passive TCP open (TCP LISTEN state), and the "client" is
   the side of the connection that did the active open.

   IAC WILL AUTHENTICATION

      The client side of the connection sends this command to indicate
      that it is willing to send and receive authentication information.

   IAC DO AUTHENTICATION

      The servers side of the connection sends this command to indicate
      that it is willing to send and receive authentication information.

   IAC WONT AUTHENTICATION

      The client side of the connection sends this command to indicate
      that it refuses to send or receive authentication information; the
      server side sends this command if it receives a DO AUTHENTICATION
      command.

   IAC DONT AUTHENTICATION

      The server side of the connection sends this command to indicate
      that it refuses to send or receive authentication information; the
      client side sends this command if it receives a WILL
      AUTHENTICATION command.

   IAC SB AUTHENTICATION SEND authentication-type-pair-list IAC SE

      The sender of this command (the server) requests that the remote
      side send authentication information for one of the authentication
      types listed in "authentication-type-pair-list".  The
      "authentication-type-pair-list" is an ordered list of
      "authentication-type" pairs.  Only the server side (DO
      AUTHENTICATION) is allowed to send this.

   IAC SB AUTHENTICATION IS authentication-type-pair <auth data> IAC SE

      The sender of this command (the client) is sending the
      authentication information for authentication type
      "authentication-type-pair".  Only the client side (WILL
      AUTHENTICATION) is allowed to send this.

   IAC SB AUTHENTICATION REPLY authentication-type-pair <auth data> IAC
   SE

      The sender of this command (the server) is sending a reply to the
      the authentication information received in a previous IS command.
      Only the server side (DO AUTHENTICATION) is allowed to send this.

   IAC SB AUTHENTICATION NAME remote-user IAC SE

      This optional command is sent to specify the account name on the
      remote host that the user wishes to be authorized to use.  Note
      that authentication may succeed, and the authorization to use a
      particular account may still fail.  Some authentication mechanisms
      may ignore this command.

   The "authentication-type-pair" is two octets, the first is the
   authentication type (as listed in Section 1, additions to this list
   must be registered with the Internet Assigned Numbers Authority
   (IANA)), and the second is a modifier to the type.  There are
   currently two one bit fields defined in the modifier, the
   AUTH_WHO_MASK bit and the AUTH_HOW_MASK bit, so there are four
   possible combinations:

      AUTH_CLIENT_TO_SERVER
      AUTH_HOW_ONE_WAY

         The client will send authentication information about the local
         user to the server.  If the negotiation is successful, the
         server will have authenticated the user on the client side of
         the connection.

      AUTH_SERVER_TO_CLIENT
      AUTH_HOW_ONE_WAY

         The server will authenticate itself to the client.  If the
         negotiation is successful, the client will know that it is
         connected to the server that it wants to be connected to.

      AUTH_CLIENT_TO_SERVER
      AUTH_HOW_MUTUAL

         The client will send authentication information about the local
         user to the server, and then the server will authenticate
         itself to the client.  If the negotiation is successful, the
         server will have authenticated the user on the client side of
         the connection, and the client will know that it is connected
         to the server that it wants to be connected to.

      AUTH_SERVER_TO_CLIENT
      AUTH_HOW_MUTUAL

         The server will authenticate itself to the client, and then the
         client will authenticate itself to the server.  If the
         negotiation is successful, the client will know that it is
         connected to the server that it wants to be connected to, and

         the server will know that the client is who it claims to be.

3.  Default Specification

   The default specification for this option is

      WONT AUTHENTICATION
      DONT AUTHENTICATION

   meaning there will not be any exchange of authentication information.

4.  Motivation

   One of the deficiencies of the Telnet protocol is that in order to
   log into remote systems, users have to type their passwords, which
   are passed in clear text through the network.  If the connections
   goes through untrusted networks, there is the possibility that
   passwords will be compromised by someone watching the packets as they
   go by.

   The purpose of the AUTHENTICATION option is to provide a framework
   for the passing of authentication information through the TELNET
   session.  This means that: 1) the users password will not be sent in
   clear text across the network, and 2) if the front end telnet process
   has the appropriate authentication information, it can automatically
   send the information, and the user will not have to type any
   password.

   It is intended that the AUTHENTICATION option be general enough that
   it can be used to pass information for any authentication system.

5.  Security Implications

   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.

6.  Implementation Rules

   WILL and DO are used only at the beginning of the connection to
   obtain and grant permission for future negotiations.

   The authentication is only negotiated in one directions; the server
   must send the "DO", and the client must send the "WILL".  This

   restriction is due to the nature of authentication; there are three
   possible cases; server authenticates client, client authenticates
   server, and server and client authenticate each other.  By only
   negotiating the option in one direction, and then determining which
   of the three cases is being used via the suboption, potential
   ambiguity is removed.  If the server receives a "DO", it must respond
   with a "WONT".  If the client receives a "WILL", it must respond with
   a "DONT".

   Once the two hosts have exchanged a DO and a WILL, the server is free
   to request authentication information.  In the request, a list of
   supported authentication types is sent.  Only the server may send
   requests ("IAC SB AUTHENTICATION SEND authentication-type-pair-list
   IAC SE").  Only the client may transmit authentication information
   via the "IAC SB AUTHENTICATION IS authentication-type ... IAC SE"
   command.  Only the server may send replys ("IAC SB AUTHENTICATION
   REPLY authentication-type ... IAC SE").  As many IS and REPLY
   suboptions may be exchanged as are needed for the particular
   authentication scheme chosen.

   If the client does not support any of the authentication types listed
   in the authentication-type-pair-list, a type of NULL should be used
   to indicate this in the IS reply.  Note that in this case, the server
   may choose to close the connection.

   The order of the authentication types MUST be ordered to indicate a
   preference for different authentication types, the first type being
   the most preferred, and the last type the least preferred.

   The following is an example of use of the option:

       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 Kerberos authentication, but is
         willing to do just one-way Kerberos 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 "joe"
       IAC SE
       IAC SB AUTHENTICATION IS
       KERBEROS_V4 CLIENT|MUTUAL AUTH 4

       7 1 67 82 65 89 46 67 7 9 77 0
       48 24 49 244 109 240 50 208 43
       35 25 116 104 44 167 21 201 224
       229 145 20 2 244 213 220 33 134
       148 4 251 249 233 229 152 77 2
       109 130 231 33 146 190 248 1 9
       31 95 94 15 120 224 0 225 76 205
       70 136 245 190 199 147 155 13
       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 REPLY
       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

   It is expected that any implementation that supports the Telnet
   AUTHENTICATION option will support all of this specification.

7.  References

   [1] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
       USC/Information Sciences Institute, July 1992.

Security Considerations

   Security issues are discussed in Section 5.

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