Telnet Authentication Option

Network Working Group                                     T. Ts'o, Editor
Request for Comments: 2941                               VA Linux Systems
Obsoletes: 1416                                                 J. Altman
Category: Standards Track                             Columbia University
                                                           September 2000


                      Telnet Authentication Option

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes the authentication option to the telnet [1]
   protocol as a generic method for negotiating an authentication type
   and mode including whether encryption should be used and if
   credentials should be forwarded.  While this document summarizes
   currently utilized commands and types it does not define a specific
   authentication type.  Separate documents are to be published defining
   each authentication type.

   This document updates a previous specification of the telnet
   authentication option, RFC 1416 [2], so that it can be used to
   securely enable the telnet encryption option [3].

1.  Command Names and Codes

      AUTHENTICATION          37

          Authentication Commands
          IS                       0
          SEND                     1
          REPLY                    2
          NAME                     3

          Authentication Types
          NULL                     0
          KERBEROS_V4              1
          KERBEROS_V5              2
          SPX*                     3
          MINK*                    4
          SRP                      5
          RSA*[also used by SRA*]  6
          SSL*                     7
          [unassigned]             8
          [unassigned]             9
          LOKI*                   10
          SSA*                    11
          KEA_SJ                  12
          KEA_SJ_INTEG            13
          DSS                     14
          NTLM*                   15

       Authentication types followed by (*) were never submitted to the
       IETF for consideration as an Internet standard.

       Following historical practice, future authentication type numbers
       and authentication modifiers will be assigned by the IANA under a
       First Come First Served policy as outlined by RFC 2434 [4].
       Despite the fact that authentication type numbers are allocated
       out of an 8-bit number space (as are most values in the telnet
       specification) it is not anticipated that the number space is or
       will become in danger of being exhausted.  However, if this
       should become an issue, when over 50% of the number space becomes
       allocated, the IANA shall refer allocation requests to either the
       IESG or a designated expert for approval.  IANA is instructed not
       to issue new suboption values without submission of documentation
       of their use.

          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

          ENCRYPT_MASK        20
          ENCRYPT_OFF              0
          ENCRYPT_USING_TELOPT     4
          ENCRYPT_AFTER_EXCHANGE  16
          ENCRYPT_RESERVED        20

          INI_CRED_FWD_MASK    8
          INI_CRED_FWD_OFF         0
          INI_CRED_FWD_ON          8

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 performed 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 must send 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 must send 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, and the second is a modifier to the type.  The
   authentication type may or may not include built-in encryption.  For
   instance, when the Kerberos 4 authentication type is negotiated
   encryption must be negotiated with the telnet ENCRYPT option.
   However, the SSL and KEA_SJ authentication types provide an encrypted
   channel as part of a successful telnet AUTH option negotiation.

   There are currently five one bit fields defined in the modifier.  The
   first two of these bits are processed as a pair, the AUTH_WHO_MASK
   bit and the AUTH_HOW_MASK bit.  There are four possible combinations
   of these two bits:

      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.

      The third and fifth bits in the modifier are the ENCRYPT_MASK
      bits.  These bits are used to determine if and how encryption
      should be enabled.  Of the four possible combinations only three
      are currently defined:

         ENCRYPT_OFF

            Encryption will not be used for this session.  TELOPT
            ENCRYPT SHOULD NOT be negotiated.  This mode MUST be used
            with all AUTH types that do not provide a shared secret to
            be used as a session key.

         ENCRYPT_USING_TELOPT

            Encryption will be negotiated via the use of TELOPT ENCRYPT.
            Immediately after authentication has completed TELOPT
            ENCRYPT MUST be negotiated in both directions.  This is
            required to occur before credentials forwarding; other
            telnet options are negotiated; or any user data is
            transmitted.  A failure to successfully negotiate TELOPT
            ENCRYPT in either direction MUST result in immediate session
            termination.

         ENCRYPT_AFTER_EXCHANGE

            Encryption will be activated in both directions immediately
            after the successful exchange of the shared secret to be
            used as the session key.  The encryption algorithm to be
            used MUST be implied by the AUTH type.

      The fourth bit field in the modifier is the INI_CRED_FWD_MASK bit.
      This bit is either set to INI_CRED_FWD_ON or INI_CRED_FWD_OFF.
      This bit is set by the client to advise the server to expect
      forwarded credentials from the client.

         INI_CRED_FWD_OFF

            The client will not be forwarding credentials to the server.
            This mode must be used if the selected authentication method
            does not support credentials forwarding.

         INI_CRED_FWD_ON

            Once authentication, and perhaps encryption, completes, the
            client will immediately forward authentication credentials
            to the server.

      The motivation for this advisory bit is that the server may wish
      to wait until the forwarded credentials have been sent before
      starting any operating system specific login procedures which may
      depend on these credentials.  Note that credentials forwarding may
      not be supported by all authentication mechanisms.  It is a
      protocol error to set this bit if the underlying authentication
      mechanism does not support credentials forwarding.

      Credentials forwarding MUST NOT be performed if
      AUTH_CLIENT_TO_SERVER|AUTH_HOW_ONE_WAY was used since the identity
      of the server can not be assured.  Credentials SHOULD NOT be
      forwarded if the telnet connection is not protected using some
      encryption or integrity protection services.

      Note that older implementations of the telnet authentication
      option will not understand the ENCRYPT_MASK and INI_CRED_FWD_MASK
      bits.  Hence an implementation wishing to offer these bits should
      offer authentication type pairs with these bits both set and not
      set if backwards compatibility is required.

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 go
   through untrusted networks, there is the possibility that passwords
   will be compromised by someone watching the packets while in transit.

   The purpose of the AUTHENTICATION option is to provide a framework
   for the passing of authentication information through the TELNET
   session, and a mechanism to enable encryption of the data stream as a
   side effect of successful authentication or via subsequent use of the
   telnet ENCRYPT option.  This means that: 1) the users password will
   not be sent in clear text across the network, 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.  3) once authentication has succeeded, the data
   stream can be encrypted to provide protection against active attacks.

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

   If the authentication type requires that encryption be enabled as a
   separate optional negotiation (the ENCRYPT option), it will provide a
   window of vulnerability from when the authentication completes, up to
   and including the negotiation to turn on encryption by an active
   attacker.  An active attack is one where the underlying TCP stream
   can be modified or taken over by the active attacker.  If the server
   only offers authentication type pairs that include the
   ENCRYPT_USING_TELOPT set in the ENCRYPT_MASK field, this will avoid
   the window of vulnerability, since both parties will agree that
   telnet ENCRYPT option must be successfully negotiated immediately
   following the successful completion of telnet AUTH.

   Other authentication types link the enabling of encryption as a side
   effect of successful authentication.  This will also provide
   protection against the active attacker.  The ENCRYPT_AFTER_EXCHANGE
   bit allows these authentication types to negotiate encryption so that
   it can be made optional.

   Another opportunity for active attacks is presented when encryption
   may be turned on and off without re-authentication.  Once encryption
   is disabled, an attacker may hijack the telnet stream, and interfere
   with attempts to restart encryption.  Therefore, a client SHOULD NOT
   support the ability to turn off encryption.  Once encryption is
   disabled, if an attempt to re-enable encryption fails, the client
   MUST terminate the telnet connection.

   It is important that in both cases the authentication type pair be
   integrity protected at the end of the authentication exchange.  This
   must be specified for each authentication type to ensure that the
   result of the telnet authentication option negotiation is agreed to
   by both the client and the server.  Some authentication type
   suboptions may wish to include all of the telnet authentication
   negotiation exchanges in the integrity checksum, to fully protect the
   entire exchange.

   Each side MUST verify the consistency of the auth-type-pairs in each
   message received.  Any variation in the auth-type-pair MUST be
   treated as a fatal protocol error.

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 direction; 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 replies ("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 if the client responds
   with a type of NULL, the server may choose to close the connection.

   When the server has concluded that authentication cannot be
   negotiated with the client it should send IAC DONT AUTH to the
   client.

   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.

   As long as the server is WILL AUTH it may request authentication
   information at any time.  This is done by sending a new list of
   supported authentication types.  Requesting authentication
   information may be done as a way of verifying the validity of the
   client's credentials after an extended period of time or to negotiate
   a new session key for use during encryption.

7.  User Interface

   Normally protocol specifications do not address user interface
   specifications.  However, due to the fact that the user will probably
   want to be able to configure the authentication and encryption and
   know whether or not the negotiations succeeded, some guidance needs
   to be given to implementors to provide some minimum level of user
   control.

   The user MUST be able to specify whether or not authentication is to
   be used, and whether or not encryption is to used if the
   authentication succeeds.  There SHOULD be at least four settings,
   REQUIRE, PROMPT, WARN and DISABLE.  Setting the authentication switch
   to REQUIRE means that if the authentication fails, then an
   appropriate error message must be displayed and the TELNET connection
   must be terminated.  Setting the authentication switch to PROMPT
   means that if the authentication fails, then an appropriate error
   message must be displayed and the user must be prompted for
   confirmation before continuing the TELNET session.  Setting the
   authentication switch to WARN means that if the authentication fails,
   then an appropriate error message must be displayed before continuing
   the TELNET session.  Setting the authentication switch to DISABLE
   means that authentication will not be attempted.  The encryption
   switch SHOULD have the same settings as the authentication switch;
   however its settings are only used when authentication succeeds.  The
   default setting for both switches should be WARN.  Both of these
   switches may be implemented as a single switch, though having them
   separate gives more control to the user.

8.  Example

   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 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

   The following is an example of use of the option with encryption
   negotiated via telnet ENCRYPT:

   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|ENCRYPT_USING_TELOPT
                                    KERBEROS_V4 CLIENT|ONE_WAY IAC
                                    SE
   [ The server has requested mutual Kerberos authentication, but is
     willing to do just one-way Kerberos authentication.  In both
     cases it is willing to encrypt the data stream.  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|ENCRYPT_USING_TELOPT
   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|ENCRYPT_USING_TELOPT
                                    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|ENCRYPT_USING_TELOPT
   CHALLENGE xx xx xx xx xx xx xx
   xx IAC SE
   [ The server sends across a RESPONSE to prove that it really is
     the right server.  ]
                                    IAC SB AUTHENTICATION REPLY
                                    KERBEROS_V4
                                    CLIENT|MUTUAL|ENCRYPT_USING_TELOPT
                                    RESPONSE yy yy yy yy yy yy yy yy
                                    IAC SE
   [ At this point, the client and server begin to negotiate the
     telnet ENCRYPT option in each direction for a secure channel.
     If the option fails in either direction for any reason the
     connection must be immediately terminated.  ]

   The following is an example of use of the option with integrated
   encryption:

   Client                           Server
                                    IAC DO AUTHENTICATION
   IAC WILL AUTHENTICATION
   [ The server is now free to request authentication information.
     ]
                                    IAC SB AUTHENTICATION SEND
                                    KEA_SJ
                                    CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
                                    IAC SE
   [ The server has requested mutual KEA authentication with
     SKIPJACK encryption.  The client will now respond with the name
     of the user that it wants to log in as and the KEA cert.  ]
   IAC SB AUTHENTICATION NAME "joe"
   IAC SE IAC SB AUTHENTICATION IS
   KEA_SJ
   CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
   '1' CertA||Ra IAC SE
   [ The server responds with its KEA Cert.  ]
                                    IAC SB AUTHENTICATION REPLY
                                    KEA_SJ
                                    CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
                                    '2'
                                    CertB||Rb||IVb||Encrypt(NonceB)
                                    IAC SE
   [ Next, the client sends across a CHALLENGE to verify that it is
     really talking to the right server.  ]
   IAC SB AUTHENTICATION IS KEA_SJ
   CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
   '3' IVa||Encrypt( NonceB xor
   0x0C18 || NonceA ) IAC SE
   [ At this point, the client begins to encrypt the outgoing data
     stream, and the server, after receiving this command, begins to
     decrypt the incoming data stream.  Lastly, the server sends
     across a RESPONSE to prove that it really is the right server.
     ]
                                    IAC SB AUTHENTICATION REPLY
                                    KEA_SJ
                                    CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
                                    '4' Encrypt( NonceA xor 0x0C18 )
                                    IAC SE
   [ At this point, the server begins to encrypt its outgoing data
     stream, and the client, after receiving this command, begins to
     decrypt its incoming data stream.  ]

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

9.  Security Considerations

   This memo describes a general framework for adding authentication and
   encryption to the telnet protocol.  The actual authentication
   mechanism is described in the authentication suboption
   specifications, and the security of the authentication option is
   dependent on the strengths and weaknesses of the authentication
   suboption.

   It should be noted that the negotiation of the authentication type
   pair is not protected, thus allowing an attacker to force the result
   of the authentication to the weakest mutually acceptable method.
   (For example, even if both sides of the negotiation can accept a
   "strong" mechanism and a "40-bit" mechanism, an attacker could force
   selection of the "40-bit" mechanism.)  An implementation should
   therefore only accept an authentication mechanism to be negotiated if
   it is willing to trust it as being secure.

   It should also be noted that the negotiation of the username in the
   IAC SB AUTHENTICATION NAME name IAC SE message is not protected.
   Implementations should verify the value by a secure method before
   using this untrusted value.

11.  Acknowledgements

   Many people have worked on this document over the span of many years.
   Dave Borman was a document editor and author of much of the original
   text.  Other folks who have contributed ideas and suggestions to this
   text include: David Carrel, Jeff Schiller, and Richard Basch.

10.  References

   [1] Postel, J. and J. Reynolds, "Telnet Protocol Specification", STD
       8, RFC 854, May 1983.

   [2] Borman D., "Telnet Authentication Option", RFC 1416, February
       1993.

   [3] Ts'o, T., "Telnet Data Encryption Option", RFC 2946, September
       2000.

   [4] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

12.  Authors' Addresses

   Theodore Ts'o, Editor
   VA Linux Systems
   43 Pleasant St.
   Medford, MA 02155

   Phone: (781) 391-3464
   EMail: tytso@mit.edu


   Jeffrey Altman
   Columbia University
   Watson Hall Room 716
   612 West 115th Street
   New York NY 10025

   Phone: +1 (212) 854-1344
   EMail: jaltman@columbia.edu


   Mailing List: telnet-wg@BSDI.COM

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