





Network Working Group                    Internet Engineering Task Force
Internet-Draft                                      Telnet Working Group
                                                       D. Borman, Editor
                                                     Cray Research, Inc.
                                                           February 1991


                      TTeellnneett AAuutthheennttiiccaattiioonn OOppttiioonn

Status of this Memo

   This draft document will be submitted to the RFC editor as a protocol
   specification.  Distribution of this memo is unlimited.  Please send
   comments to the telnet-ietf@cray.com mailing list.

1.  Command Names and Codes

   AUTHENTICATION TBA
       IS               0
       SEND             1
       REPLY            2

       Authentication Types
       NULL             0
       KERBEROS_V4      1
       KERBEROS_V5      2
       SPX              3
       MINK             4

       Modifiers
       AUTH_WHO_CLIENT      0
       AUTH_WHO_SERVER      1
       AUTH_WHO_MASK        1

       AUTH_HOW_ONE_WAY     0
       AUTH_HOW_MUTUAL      2
       AUTH_HOW_MASK        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, and the "client" is the side of the
   connection that did the active open.

   IAC WILL AUTHENTICATION




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      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 AUTHENTICA-
      TION command.

   IAC SB AUTHENTICATION SEND authentication-type-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-list".  The "authentication-
      type-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 ... IAC SE

      The sender of this command (the client) is sending the authentica-
      tion 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 ... 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.


   The "authentication-type-pair" has two values, the first is the au-
   thentication type, 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 possi-
   ble combinations:

      AUTH_WHO_SERVER



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      AUTH_HOW_ONE_WAY
         The server is authenticating the client.

      AUTH_WHO_CLIENT
      AUTH_HOW_ONE_WAY
         The client is authenticating the server.

      AUTH_WHO_SERVER
      AUTH_HOW_MUTUAL
         The client and server are authenticating each other, and the
         client will begin the negotiation.

      AUTH_WHO_CLIENT
      AUTH_HOW_MUTUAL
         The client and server are authenticating each other, and the
         server will begin the negotiation.

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 deficiences 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 ses-
   sion.  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 pass-
   word.

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

5.  Implementation Rules

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




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   The authentication is only negotiated in one directions; the server
   must send the "DO", and the client must send the "WILL".  This 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 re-
   quests ("IAC SB AUTHENTICATION SEND authentication-type-list IAC
   SE").  Only the client may transmit authentication information via
   the "IAC SB AUTHENTICATION IS authentication-type ... IAC SE" com-
   mand.  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-list, a type of NULL should be used to in-
   dicate 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:

       _C_l_i_e_n_t                           _S_e_r_v_e_r
                                        IAC DO AUTHENTICATION
       IAC WILL AUTHENTICATION
       [ The server is now free to request authentication information.
         ]
                                        IAC SB AUTHENTICATION SEND
                                        KERBEROS_V4 SERVER|ONE_WAY IAC
                                        SE
       [ The server has requested 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.  A new key is also sent,
         for possible later user by the ENCRYPT option.  ]
       IAC SB AUTHENTICATION IS
       KERBEROS_V4 SERVER|ONE_WAY NAME
       "joe" IAC SE
       IAC SB AUTHENTICATION IS KRB4
       SERVER|ONE_WAY AUTH 4 7 1 67 82
       65 89 46 67 7 9 77 0 48 24 49



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       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
       IAC SB AUTHENTICATION IS
       KERBEROS_V4 SERVER|ONE_WAY NEW-
       KEY 170 243 56 226 94 230 157 10
       IAC SE
       [ The server responds with an ACCEPT command to state that the
         authentication was successful.  ]
                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V4 SERVER|ONE_WAY AC-
                                        CEPT "dab"

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

6. Kerberos Version 4 and 5 Definitions

   Command Names and Codes

       AUTH     0
       REJECT   1
       ACCEPT   2
       NEWKEY   3
       NAME     4

   All of these commands take the form IAC SB AUTHENTICATION KERBEROS_V4
   SERVER|ONE_WAY <sub-subcommand> IAC SE.  The following describes the
   format for <sub-subcommand>.

   Command Meanings

      NAME <principal to authorize as>

         This is the Kerberos principal that you wish to be authorized
         as.

      AUTH <kerberos ticket>

         This is used to pass the Kerberos ticket to the remote side of
         the connection.

      NEWKEY <eight octet encrypted DES key>

         This command is used by the client to pass a new DES key to the
         server.  The client generates a random eight octet DES key,



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         (called the encrypt key), uses DES to encrypt it with the ses-
         sion key, and uses the NEWKEY command to send it to the server.
         The server decrypts it using the session key.  This is not used
         in the authentication, but it provides a unique key that may
         later be used with the KRBDES type of encryption, as specified
         by the telnet ENCRYPT option.

      ACCEPT <authenticated principal>

         This command indicates that the authentication was successful,
         and optionally the principal that the client has been authenti-
         cated as.

      REJECT <optional reason for rejection>

         This command indicates that the authentication was not success-
         ful, and if there is any more data in the sub-option, it is an
         ASCII text message of the reason for the rejection.


   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 ac-
   count, then the client would send NAME "pete" AUTH <joe's kerberos
   ticket> The server would then authenticate the user as "joe" from the
   ticket information, since "pete" is allowing "joe" to use his ac-
   count, the server would send back ACCEPT "joe".

   The commands for Kerberos Version 5 are the same as the commands for
   Kerberos Version 4.

7.  References


   [1] Reynolds, Joyce, and Postel, Jon, "Assigned Numbers", RFC 1060,
       ISI, March 1990

Author's Address

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

   Phone: (612) 452-6650

   Mailing List: telnet-ietf@CRAY.COM
   EMail: dab@CRAY.COM






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