Internet Info 1994 March/Internet Info CD-ROM (Walnut Creek) (March 1994).iso/inet/internet-drafts/draft-ietf-telnet-encryption-00.txt

Text File | 1993-04-08 | 24KB | 580 lines

Network Working Group                    Internet Engineering Task Force
Internet-Draft                                      Telnet Working Group
                                                       D. Borman, Editor
                                                     Cray Research, Inc.
                                                              April 1993


              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 draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress."

   Please check the 1id-abstracts.txt listing contained in the
   internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net,
   nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the
   current status of any Internet Draft.

1.  Command Names and Codes

   AUTH_ENCRYPT            37
       Authentication Commands
       IS                       0
       SEND                     1
       REPLY                    2
       NAME                     3
       END_ENCRYPT              4
       REQUEST_END_ENCRYPT      5

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

       Modifiers



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       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

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 AUTH_ENCRYPT

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

   IAC DO AUTH_ENCRYPT

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

   IAC WONT AUTH_ENCRYPT

      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 AUTH_ENCRYPT
      command.

   IAC DONT AUTH_ENCRYPT

      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 AUTH_ENCRYPT
      command.

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

      The sender of this command (the server) requests that the remote



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      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
      AUTH_ENCRYPT) is allowed to send this.

   IAC SB AUTH_ENCRYPT IS authentication-type-pair <auth data> 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 AUTH_ENCRYPT) is allowed to
      send this.

   IAC SB AUTH_ENCRYPT 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 AUTH_ENCRYPT) is allowed to send this.

   IAC SB AUTH_ENCRYPT 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.

   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.

   IAC SB AUTH_ENCRYPT REQUEST_END_ENCRYPT IAC SE

      The sender of this command requests that the remote side stop en-
      cryption of the telnet data stream.  This command is advisory
      only.  See the "Implementation Rules" section for more details.


   The "authentication-type-pair" is two octets, the first is the au-
   thentication type, and the second is a modifier to the type.  There
   are currently three 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



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         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 con-
         nected 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 it-
         self 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 negotia-
         tion 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 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 authentica-
      tion completes, the client will immediately forward authentication
      credentials to the server.  This bit is set by the client to ad-
      vise 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 mechanisms.  It is a proto-
      col error to set this bit if the underlying authentication mechan-
      ism does not support credentials forwarding.



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3.  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 encryp-
   tion 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 AUTH_ENCRYPT 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 ef-
   fect of successful authentication.  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 informa-
   tion, 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 AUTH_ENCRYPT 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 in-
   truder 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 the active attacker.  If encryption were en-
   abled as a separate negotiation, it would provide a window of vulner-
   ability from when the authentication completes, up to and including
   the negotiation to turn on encryption.  It is because of this that,



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   while there is a command to end encryption, there is not a command to
   restart encryption.  The only safe way to restart encryption once it
   has been turned off, is to repeat the entire authentication
   processes.

6.  Implementation Rules

   WILL and DO are used only at the beginning of the connection to ob-
   tain 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 res-
   triction is due to the nature of authentication; there are three pos-
   sible cases; server authenticates client, client authenticates
   server, and server and client authenticate each other.  By only nego-
   tiating 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 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 replys ("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
   scheme chosen.

   When determining a match from the authentication-type-pair-list re-
   ceived fromt the server, the client may ignore the AUTH_ENCRYPT_MASK
   bit.  If the AUTH_ENCRYPT_MASK bit was ENCRYPT_OFF, then the client
   may only 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 is 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, a type of NULL should be used
   to indicate this in the IS reply.  Note that if the client turns of
   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
   immediatly following the "IAC SB AUTH_ENCRYPT REPLY ... IAC SE" com-
   mand that signifies that the server has successfully completed the



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   authentication process.  Encryption from the client to the server be-
   gins with the first byte immediatly 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 authenti-
   cation and encryption type.  All data, including TELNET options, are
   encrypted.

   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.

   When using END_ENCRYPT or REQUEST_END_ENCRYPT, care must be used.
   The normal scenario in which one would want to turn of encryption for
   performance reasons is when just the output stream from the server to
   the client is sent in the clear (which has the bulk of the data), and
   the input stream from the client to the server is left encrypted (so
   that typed passwords are not readable).  The client would send a
   REQUEST_END_ENCRYPT command to the server, who would 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 output data stream 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 probably be ignored if received over a clear-text
   data stream.  If you wish to disable all encryption, you should first
   send a REQUEST_END_ENCRYPT to get the other side to stop encrypting
   its data, and then send the END_ENCRYPT and stop encrypting your data
   stream.

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 specify the things about authentication and en-
   cryption and also know whether or not things 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 authentica-
   tion succeeds.  There should be at least four settings, REQUIRE,
   PROMPT, WARN and DISABLE.  Setting the authentication switch to RE-
   QUIRE means that if the authentication fails, then an appropriate er-
   ror message must be displayed and the TELNET connection must be ter-
   minated.  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 con-
   tinuing the TELNET session.  Setting the authentication switch to
   WARN means that if the authentication fails, then an appropriate er-



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   ror message must be displayed before continuing the TELNET session.
   Setting the authentication switch to DISABLE means that authentica-
   tion 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
   switchs should be WARN.  Both of these switchs may be implemented as
   a single switch, though having them seperate gives more control to
   the user.

8.  Example

   The following is an example of use of the option:

       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 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 AUTH_ENCRYPT NAME "joe"
       IAC SE
       IAC SB AUTH_ENCRYPT 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 AUTH_ENCRYPT 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 AUTH_ENCRYPT REPLY
       KERBEROS_V4 CLIENT|MUTUAL
       CHALLENGE xx xx xx xx xx xx xx
       xx IAC SE



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       [ Lastly, the server sends across a RESPONSE to prove that it
         really is the right server.
                                        IAC SB AUTH_ENCRYPT 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:

       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
                                        ENCRYPT_ON|CLIENT|MUTUAL
                                        KERBEROS_V4
                                        ENCRYPT_ON|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 AUTH_ENCRYPT NAME "joe"
       IAC SE
       IAC SB AUTH_ENCRYPT IS
       KERBEROS_V4
       ENCRYPT_ON|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 AUTH_ENCRYPT REPLY
                                        KERBEROS_V4
                                        CLIENT|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 REPLY
       KERBEROS_V4 CLIENT|MUTUAL
       CHALLENGE xx xx xx xx xx xx xx
       xx IAC SE



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       [ 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 CLIENT|MUTUAL
                                        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.  ]

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

9.  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: [phone]

   Mailing List: [e-mail]
   EMail: [e-mail]

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: [phone] x256
   EMail: [e-mail]







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