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+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
+
+
+
+Ts'o & Altman               Standards Track                     [Page 1]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+          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
+
+
+
+Ts'o & Altman               Standards Track                     [Page 2]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+          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.
+
+
+
+Ts'o & Altman               Standards Track                     [Page 3]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+   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
+
+
+
+Ts'o & Altman               Standards Track                     [Page 4]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+         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.
+
+
+
+Ts'o & Altman               Standards Track                     [Page 5]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+         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.
+
+
+
+
+Ts'o & Altman               Standards Track                     [Page 6]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+   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
+
+
+
+
+Ts'o & Altman               Standards Track                     [Page 7]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+   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.
+
+
+
+Ts'o & Altman               Standards Track                     [Page 8]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+   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.
+
+
+
+
+
+
+
+
+Ts'o & Altman               Standards Track                     [Page 9]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+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
+
+
+
+Ts'o & Altman               Standards Track                    [Page 10]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+   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
+
+
+
+
+
+
+
+
+Ts'o & Altman               Standards Track                    [Page 11]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+   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
+
+
+
+
+Ts'o & Altman               Standards Track                    [Page 12]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+   [ 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.
+
+
+
+
+
+Ts'o & Altman               Standards Track                    [Page 13]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+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
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Ts'o & Altman               Standards Track                    [Page 14]
+
+RFC 2941              Telnet Authentication Option        September 2000
+
+
+13.  Full Copyright Statement
+
+   Copyright (C) The Internet Society (2000).  All Rights Reserved.
+
+   This document and translations of it may be copied and furnished to
+   others, and derivative works that comment on or otherwise explain it
+   or assist in its implementation may be prepared, copied, published
+   and distributed, in whole or in part, without restriction of any
+   kind, provided that the above copyright notice and this paragraph are
+   included on all such copies and derivative works.  However, this
+   document itself may not be modified in any way, such as by removing
+   the copyright notice or references to the Internet Society or other
+   Internet organizations, except as needed for the purpose of
+   developing Internet standards in which case the procedures for
+   copyrights defined in the Internet Standards process must be
+   followed, or as required to translate it into languages other than
+   English.
+
+   The limited permissions granted above are perpetual and will not be
+   revoked by the Internet Society or its successors or assigns.
+
+   This document and the information contained herein is provided on an
+   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Ts'o & Altman               Standards Track                    [Page 15]
+