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Versions: 00                                                            
Network Working Group                                          J. Galvin
INTERNET-DRAFT                                                 S. Murphy
draft-galvin-telnet-authenc-00.txt                           D. Balenson
                                                                     TIS
                                                               July 1995


              Telnet Authentication and Encryption Option



Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are valid for a maximum of six months and may be
   updated, replaced, or obsoleted by other documents at any time.  It
   is inappropriate to use Internet Drafts as reference material or to
   cite them other than as ``work in progress''.

   To learn the current status of any Internet Draft, please check the
   1id-abstracts.txt listing contained in one of the Internet Drafts
   Shadow Directories on ds.internic.net (US East Coast), venera.isi.edu
   (US West Coast), munnari.oz.au (Pacific Rim), or nic.nordu.net
   (Europe).

Abstract

   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 the clear through the network.  This document specifies the
   AUTH_ENCRYPT option, whose purpose is two-fold: to provide a
   framework for the passing of authentication information through the
   TELNET session and to provide a mechanism to enable encryption of the
   data stream as a side effect of successful authentication.

Acknowledgements

   This document represents a revision of RFC1416 [1] that has been
   enhanced to include the optional encryption of the data stream.  The
   work of Dave Borman (Editor), Steve Alexander (Telnet working group
   chair), and the Telnet working group in the preparation of that
   document is gratefully acknowledged.  Ted Ts'o deserves special
   mention for keeping track of the details of this enhancement and



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   providing them to us so that this document could be prepared.

1.  Introduction

   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 the clear through the network.  If the connection goes
   through untrusted networks, there is the possibility that an intruder
   may eavesdrop on the packets as they go by and thus obtain the
   password.

   This document specifies the AUTH_ENCRYPT option, whose purpose is
   two-fold: to provide a framework for the passing of authentication
   information through the TELNET session and to provide a mechanism to
   enable encryption of the data stream as a side effect of successful
   authentication.  This means that:

   1) the user's password will not be sent unencrypted across the
   network,

   2) if the front end telnet process has the appropriate authentication
   information it can automatically send it instead of the user typing
   any password, and

   3) once authentication has succeeded the data stream can be encrypted
   to provide protection against active attacks.

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

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

2.  Command Names and Codes

   This section lists the codes for the Telnet authentication and
   encryption option, commands, and modifiers, as well as the initially
   defined authentication and encryption types.  The codes for the
   authentication and encryption types are officially assigned and
   maintained by IANA.  The values are published regularly in STD2,
   which is currently RFC1700 [2].

   AUTH_ENCRYPT            XX

       Authentication and Encryption Commands

       IS                       0
       SEND                     1
       REPLY                    2



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       NAME                     3
       END_ENCRYPT              4
       REQUEST_END_ENCRYPT      5

       Authentication and Encryption Types

       NULL                     0
       KERBEROS_V4              1
       KERBEROS_V5              2
       SPX                      3
       RSA                      6
       LOKI                    10
       GSSAPI                  XX

       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         4
       ENCRYPT_OFF              0
       ENCRYPT_ON               4

       INI_CRED_FWD_MASK    8
       INI_CRED_FWD_OFF         0
       INI_CRED_FWD_ON          8


3.  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 AUTH_ENCRYPT

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

   IAC DO AUTH_ENCRYPT

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



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      encryption information.

   IAC WONT AUTH_ENCRYPT

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

   IAC DONT AUTH_ENCRYPT

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

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

      The server side of the connection sends this command to request
      that the client side of the connection send authentication
      information for one of the authentication types listed in the
      "authentication-type-pair-list".  The "authentication-type-pair-
      list" is an ordered list of "authentication-type" pairs.

   IAC SB AUTH_ENCRYPT IS authentication-type-pair authentication-data
   IAC SE

      The client side of the connection sends this command to indicate
      the "authentication-type-pair" that was chosen for the connection
      from the list provided by the server side and to send the
      authentication-data necessary for it.

   IAC SB AUTH_ENCRYPT REPLY authentication-type-pair authentication-
   data IAC SE

      The server side of the connection sends this command in response
      to authentication-data in a previous IS command and to send the
      authentication-data the client needs.

   IAC SB AUTH_ENCRYPT NAME remote-user IAC SE

      The client side of the connection sends this command to indicate
      the account name (remote-user) on the server that the user wishes
      to be authorized to use.  Note that while authentication may
      succeed, the authorization to use a particular account may fail.
      Some authentication types may ignore this command.  This command
      supercedes the value of the USER environment variable if it is
      passed from the client to the server.



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   IAC SB AUTH_ENCRYPT END_ENCRYPT IAC SE

      The sender of this command is stating that at this point in the
      data stream, all following data will no longer be encrypted.  This
      command should only be sent in an encrypted data stream and should
      be ignored if received in an unencrypted data stream.  See the
      "Implementation Rules" section for more details.

   IAC SB AUTH_ENCRYPT REQUEST_END_ENCRYPT IAC SE

      The sender of this command requests that the remote side stop
      encryption of the telnet data stream.  This command is advisory
      only.  This command should only be sent in an encrypted data
      stream and should be ignored if received in an unencrypted data
      stream.  See the "Implementation Rules" section for more details.


   The "authentication-type-pair" is two octets: the first is the
   authentication type and the second is a modifier to the type.  There
   are currently four one bit fields defined in the modifier.  Two of
   these 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 to which it wants to be connected.

      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 to which it wants to be connected.

      AUTH_SERVER_TO_CLIENT



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      AUTH_HOW_MUTUAL

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

      The third bit field in the modifier is the ENCRYPT_MASK bit.  This
      bit is either set to ENCRYPT_ON or ENCRYPT_OFF.  Setting this bit
      to ENCRYPT_ON implies that once authentication completes, the data
      stream is to be encrypted in both directions using the encryption
      method specified for the authentication 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.
      Setting this bit to INI_CRED_FWD_ON implies that once
      authentication completes, the client will immediately forward
      authentication credentials to the server.  This bit is set by the
      client to advise the server to expect forwarded credentials from
      the client.

      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 types.  It is a protocol
      error to set this bit if the underlying authentication type does
      not support credentials forwarding.

      The authentication-data may be omitted if there is none to be
      provided for the type being negotiated.


4.  Default Specification

   The default specification for this option is

      WONT AUTH_ENCRYPT
      DONT AUTH_ENCRYPT

   meaning there will not be any exchange of authentication or
   encryption information.

5.  Implementation Rules

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




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   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 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 AUTH_ENCRYPT SEND authentication-type-pair-list IAC
   SE").  Only the client MAY transmit authentication information via
   the "IAC SB AUTH_ENCRYPT IS authentication-type ... IAC SE" command.
   Only the server MAY send replies ("IAC SB AUTH_ENCRYPT REPLY
   authentication-type ... IAC SE").  As many IS and REPLY suboptions
   MAY be exchanged as are needed for the particular authentication type
   chosen.

   When determining a match from the authentication-type-pair-list
   received from the server, the client MAY ignore the AUTH_ENCRYPT_MASK
   bit.  If the AUTH_ENCRYPT_MASK bit was ENCRYPT_OFF, then the client
   MUST respond with ENCRYPT_OFF.  If the AUTH_ENCRYPT_MASK bit was on,
   then the client MAY respond with either ENCRYPT_ON or ENCRYPT_OFF.
   In the latter case the client is stating that it will do
   authentication but it does not want to encrypt the data stream.

   If the client does not support any of the authentication types listed
   in the authentication-type-pair-list, it SHOULD indicate this in the
   IS reply with a type of NULL.  Note, if the client turns off the
   ENCRYPT_ON bit or responds with a type of NULL, the server MAY choose
   to close the connection.

   Encryption from the server to the client begins with the first byte
   immediately following the "IAC SB AUTH_ENCRYPT REPLY ... IAC SE"
   command that signifies that the server has successfully completed the
   authentication process.  Encryption from the client to the server
   begins with the first byte immediately following the "IAC SB
   AUTH_ENCRYPT RESPONSE ... IAC SE" command that signifies that the
   client has successfully completed the authentication process.  Both
   of these will be specified in the document for the specific
   authentication and encryption type.  All data, including TELNET
   options, are encrypted.

   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 being the least preferred.



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   Special consideration applies to the use of END_ENCRYPT and
   REQUEST_END_ENCRYPT.  A scenario during which one may want to turn
   off encryption is communication from the server to the client, which
   has the bulk of the data; leaving the communication from the client
   to the server encrypted ensures that typed passwords are not readable
   by eavesdropping.  To do this the client SHOULD send a
   REQUEST_END_ENCRYPT command to the server, who SHOULD then send an
   END_ENCRYPT command and stop encrypting the output data stream.  At
   this point, an active attacker could insert a REQUEST_END_ENCRYPT
   command in the data stream from the server to the client to try and
   get the client to stop encrypting its input stream to the server.
   So, a REQUEST_END_ENCRYPT command SHOULD always be honored if
   received within an encrypted data stream but SHOULD be ignored if
   received over an unencrypted data stream.  If it is desirable to
   disable all encryption, a REQUEST_END_ENCRYPT SHOULD be sent prior to
   the END_ENCRYPT to get the other side to stop encrypting first.

6.  User Interface Rules

   Normally protocol specifications do not address user interface
   issues.  However, due to the fact that the user should be able to
   indicate the information necessary to achieve a successful
   authentication and encryption negotiation and the user should know
   whether the authentication and encryption succeeded, some guidance
   must be given to implementors to assure a 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 MUST NOT be attempted.

   The encryption switch SHOULD have an independent set of the same
   settings as the authentication switch.  However, its settings MUST



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   only be used when authentication succeeds.

   The default setting for both switches SHOULD be WARN.  Both of these
   switchs MAY be implemented as a single switch, though having them
   separate gives more control to the user.

7.  Example

   The following is an example of the use of this option for
   authentication without encryption for Kerberos Version 4 [3]:

       Client                           Server

                                        IAC DO AUTH_ENCRYPT
       IAC WILL AUTH_ENCRYPT

       [ The server is now free to request authentication information.
         ]

                                        IAC SB AUTH_ENCRYPT SEND
                                        KERBEROS_V4
                                        AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
                                        KERBEROS_V4
                                        AUTH_CLIENT_TO_SERVER|AUTH_HOW_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 AUTH_ENCRYPT NAME "joe"
       IAC SE
       IAC SB AUTH_ENCRYPT IS
       KERBEROS_V4
       AUTH_CLIENT_TO_SERVER|AUTH_HOW_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 AUTH_ENCRYPT REPLY



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                                        KERBEROS_V4
                                        AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
                                        ACCEPT IAC SE

       [ Next, the client sends across a CHALLENGE to verify that it is
         really talking to the right server.  ]

       IAC SB AUTH_ENCRYPT IS
       KERBEROS_V4
       AUTH_CLIENT_TO_SERVER|AUTH_HOW_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 AUTH_ENCRYPT REPLY
                                        KERBEROS_V4
                                        AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
                                        RESPONSE yy yy yy yy yy yy yy yy
                                        IAC SE

   The following is an example of the use of this option for authentica-
   tion with encryption for Kerberos Version 4 [3]:

       Client                           Server

                                        IAC DO AUTH_ENCRYPT
       IAC WILL AUTH_ENCRYPT

       [ The server is now free to request authentication information.
         ]

                                        IAC SB AUTH_ENCRYPT SEND
                                        KERBEROS_V4
                                        AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
                                        |ENCRYPT_ON KERBEROS_V4
                                        AUTH_CLIENT_TO_SERVER|AUTH_HOW_ONE_WAY
                                        |ENCRYPT_ON 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 AUTH_ENCRYPT NAME "joe"
       IAC SE
       IAC SB AUTH_ENCRYPT IS
       KERBEROS_V4



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       AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL|ENCRYPT_ON
       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 AUTH_ENCRYPT REPLY
                                        KERBEROS_V4
                                        AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
                                        |ENCRYPT_ON ACCEPT IAC SE

       [ Next, the client sends across a CHALLENGE to verify that it is
         really talking to the right server.  ]

       IAC SB AUTH_ENCRYPT IS
       KERBEROS_V4
       AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL|ENCRYPT_ON
       CHALLENGE xx xx xx xx xx xx xx
       xx 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 AUTH_ENCRYPT REPLY
                                        KERBEROS_V4
                                        AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
                                        |ENCRYPT_ON RESPONSE yy yy yy yy
                                        yy yy yy yy 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.  ]

8.  Security Considerations

   The ability to negotiate a common authentication type between a
   client and a server system is a feature of the authentication option
   that should be used with caution. When the negotiation is performed
   no authentication has yet occurred. Therefore, neither system knows



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   whether it is communicating with the intended system.  An active at-
   tacker could attempt to negotiate the use of an authentication system
   which is either weak or already compromised by the intruder.

   By linking the enabling of encryption as a side effect of successful
   authentication, protection is provided against an active attacker.
   An active attack is one where the underlying TCP stream can be modi-
   fied or taken over by an active attacker.  If encryption were enabled
   as a separate negotiation, it would provide a window of vulnerability
   from when the authentication completes up to and including the nego-
   tiation to turn on encryption.  It is because of this there is no
   command to restart encryption.  The only safe way to restart encryp-
   tion once it has been turned off is to repeat the entire authentica-
   tion process.

9.  References

   [1] D. Borman, Editor.  Telnet Authentication Option.  RFC1416, Cray
   Research, Inc., February 1993.

   [2] J. Reynolds, J. Postel.  Assigned Numbers. RFC1700, ISI, October
   1994.

   [3] D. Borman, Editor.  Telnet Authentication: Kerberos Version 4.
   RFC1411, Cray Research, Inc., January 1993.

10. Authors' Address

   Jim Galvin <galvin@tis.com>
   Sandy Murphy <murphy@tis.com>
   Dave Balenson <balenson@tis.com>

   Trusted Information Systems
   3060 Washington Road
   Glenwood, MD 21738

   Phone: 301.854.6889
















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