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Versions: (RFC 2942) 00 01 02

Network Working Group                                     Jeffrey Altman
Internet-Draft: draft-altman-telnet-rfc2942bis-02
Obsoletes: 2942
Expires:  16 July 2007
                                                       December 15, 2006


               Telnet Authentication: Kerberos Version 5

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Copyright Notice

   Copyright (C) The IETF Trust (2006).

Abstract

   This document describes how Kerberos Version 5 [RFC 4120] is used
   with the Telnet protocol [RFC 854].   It describes an Telnet
   Authentication suboption to be used with the Telnet Authentication
   option [RFC YYYY].   This mechanism can also used to provide keying
   material to provide data confidentiality services in conjunction
   with the Telnet Encryption option [RFC 2946].

   This document updates a previous specification of the Telnet
   Authentication Kerberos 5 method, RFC 2942 [4], to allow Kerberos 5
   Telnet authentication to be used in conjunction with the START_TLS
   option [RFC XXXX].


Conventions used in this document

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
  NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
  "OPTIONAL" in this document are to be interpreted as described in
  RFC 2119.

1. Introduction

   This document describes how Kerberos Version 5 [RFC 4120] is used
   with the Telnet protocol [RFC 854].   It describes an Telnet
   Authentication suboption to be used with the Telnet Authentication
   option [RFC YYYY].   This mechanism can also used to provide keying
   material to provide data confidentiality services in conjunction
   with the Telnet Encryption option [RFC 2946].

   This document updates a previous specification of the Telnet
   Authentication Kerberos 5 method, RFC 2942 [4], to allow Kerberos 5
   Telnet authentication to be used in conjunction with the START_TLS
   option [RFC XXXX].

2. Command Names and Codes

      Authentication Types

         KERBEROS_V5    2

      Sub-option Commands

         AUTH               0
         REJECT             1
         ACCEPT             2
         RESPONSE           3
         FORWARD            4
         FORWARD_ACCEPT     5
         FORWARD_REJECT     6
         TLS_VERIFIER       7

3.  Command Meanings

   IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH
   <KRB_AP_REQ message> IAC SE

      This is used to pass the Kerberos V5 [RFC 4120] KRB_AP_REQ message
      to the remote side of the connection.  The first octet of the
      <authentication-type-pair> value is KERBEROS_V5, to indicate that
      Version 5 of Kerberos is being used.

      The Kerberos V5 authenticator in the KRB_AP_REQ message must
      contain a Kerberos V5 checksum of the two-byte authentication-
      type-pair.  If the ENCRYPT_START_TLS bit mask is in use, the
      checksum must concatenate the TLS Client Finished Message and TLS
      Server Finished Message [RFC 4346] to the authentication-type-pair.
      This checksum must be verified by the server to assure that the
      authentication type pair was correctly negotiated.

      The Kerberos V5 authenticator must also include the optional
      subkey field, which shall be filled in with a randomly chosen key.
      This key shall be used for encryption purposes if encryption is
      negotiated, and shall be used as the negotiated session key (i.e.,
      used as keyid 0) for the purposes of the telnet encryption option;
      if the subkey is not filled in, then the ticket session key will
      be used instead.

      If data confidentiality services is desired the ENCRYPT_US-
      ING_TELOPT flag must be set in the authentication-type-pair as
      specified in [RFC YYYY].

   IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT IAC SE

      This command indicates that the authentication was successful.

      If the AUTH_HOW_MUTUAL bit is set in the second octet of the
      authentication-type-pair, the RESPONSE command must be sent before
      the ACCEPT command is sent.

   IAC SB AUTHENTICATION REPLY <authentication-type-pair> REJECT
      <optional reason for rejection> IAC SE

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

   IAC SB AUTHENTICATION REPLY <authentication-type-pair> RESPONSE
   <KRB_AP_REP message> IAC SE

      This command is used to perform mutual authentication.  It is only
      used when the AUTH_HOW_MUTUAL bit is set in the second octet of
      the authentication-type-pair.  After an AUTH command is verified,
      a RESPONSE command is sent which contains a Kerberos V5 KRB_AP_REP
      message to perform the mutual authentication.

   IAC SB AUTHENTICATION REPLY <authentication-type-pair> TLS_VERIFY
   MK_SAFE(<server TLS finished><client TLS finished>) IAC SE

      This command is used to verify that there is no man in the middle
      attack when a Kerberos 5 authentication is performed over a TLS
      protected session.  It is only used with AUTH_ENCRYPT_START_TLS
      is set.  If the client and server's finished messages [RFC 4346]
      cannot be verified, the connection MUST be terminated.  If this
      message is not received prior to the receipt of the ACCEPT message,
      the connection must be terminated.

   IAC SB AUTHENTICATION <authentication-type-pair> FORWARD <KRB_CRED
   message> IAC SE

      This command is used to forward kerberos credentials for use by
      the remote session.  The credentials are passed as a Kerberos V5
      KRB_CRED message which includes, among other things, the forwarded
      Kerberos ticket and a session key associated with the ticket.
      Part of the KRB_CRED message is encrypted in the key previously
      exchanged for the telnet session by the AUTH suboption.

   IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_ACCEPT IAC
   SE

      This command indicates that the credential forwarding was
      successful.

   IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_REJECT
      <optional reason for rejection> IAC SE

      This command indicates that the credential forwarding was not
      successful, and if there is any more data in the suboption, it is
      an ASCII text message of the reason for the rejection.

4.  Implementation Rules

   If the second octet of the authentication-type-pair has the AUTH_WHO
   bit set to AUTH_CLIENT_TO_SERVER, then the client sends the initial
   AUTH command, and the server responds with either ACCEPT or REJECT.
   In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the
   server will send a RESPONSE before it sends the ACCEPT.

   If the second octet of the authentication-type-pair has the AUTH_WHO
   bit set to AUTH_SERVER_TO_CLIENT, then the server sends the initial
   AUTH command, and the client responds with either ACCEPT or REJECT.
   In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the
   client will send a RESPONSE before it sends the ACCEPT.

   The Kerberos principal used by the server will generally be of the
   form "host/<hostname>@realm".  That is, the first component of the
   Kerberos principal is "host"; the second component is the fully
   qualified lower-case hostname of the server; and the realm is the
   Kerberos realm to which the server belongs.

   Any Telnet IAC characters that occur in the KRB_AP_REQ or KRB_AP_REP
   messages, the KRB_CRED structure, or the optional rejection text
   string must be doubled as specified in [RFC 855].  Otherwise the
   following byte might be mis-interpreted as a Telnet command.

5.  Examples

   User "joe" may wish to log in as user "pete" on machine "foo".  If
   "pete" has set things up on "foo" to allow "joe" access to his
   account, then the client would send IAC SB AUTHENTICATION NAME "pete"
   IAC SE IAC SB AUTHENTICATION IS KERBEROS_V5 AUTH <KRB_AP_REQ_MESSAGE>
   IAC SE

   The server would then authenticate the user as "joe" from the
   KRB_AP_REQ_MESSAGE, and if the KRB_AP_REQ_MESSAGE was accepted by
   Kerberos, and if "pete" has allowed "joe" to use his account, the
   server would then continue the authentication sequence by sending a
   RESPONSE (to do mutual authentication, if it was requested) followed
   by the ACCEPT.

   If forwarding has been requested, the client then sends IAC SB
   AUTHENTICATION IS KERBEROS_V5 CLIENT|MUTUAL FORWARD <KRB_CRED
   structure with credentials to be forwarded> IAC SE.  If the server
   succeeds in reading the forwarded credentials, the server sends
   FORWARD_ACCEPT else, a FORWARD_REJECT is sent back.

       Client                           Server
                                        IAC DO AUTHENTICATION
       IAC WILL AUTHENTICATION

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

                                        IAC SB AUTHENTICATION SEND
                                        KERBEROS_V5 CLIENT|MUTUAL
                                        KERBEROS_V5 CLIENT|ONE_WAY IAC
                                        SE

       [ The server has requested mutual Version 5 Kerberos
         authentication.  If mutual authentication is not supported,
         then the server is willing to do one-way 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
       "pete" IAC SE
       IAC SB AUTHENTICATION IS
       KERBEROS_V5 CLIENT|MUTUAL AUTH
       <KRB_AP_REQ message> IAC SE

       [ Since mutual authentication is desired, the server sends across
         a RESPONSE to prove that it really is the right server.  ]

                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V5 CLIENT|MUTUAL
                                        RESPONSE <KRB_AP_REP message>
                                        IAC SE

       [ The server responds with an ACCEPT command to state that the
         authentication was successful.  ]

                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V5 CLIENT|MUTUAL ACCEPT
                                        IAC SE

       [ If so requested, the client now sends the FORWARD command to
         forward credentials to the remote site.  ]

       IAC SB AUTHENTICATION IS KER-
       BEROS_V5 CLIENT|MUTUAL
       FORWARD <KRB_CRED message> IAC
       SE

       [ The server responds with a FORWARD_ACCEPT command to state that
         the credential forwarding was successful.  ]

                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V5 CLIENT|MUTUAL
                                        FORWARD_ACCEPT IAC SE

6. Security Considerations

   As an implementation of the TELNET AUTH option [2] all of the
   Security Considerations from that RFC MUST be considered
   applicable to this sub-option.

   The selection of the random session key in the Kerberos V5
   authenticator is critical, since this key will be used for encrypting
   the telnet data stream if encryption is enabled.  It is strongly
   advised that the random key selection be done using cryptographic
   techniques that involve the Kerberos ticket's session key.  For
   example, using the current time, encrypting it with the ticket
   session key, and then correcting for key parity is a strong way to
   generate a subsession key, since the ticket session key is assumed to
   be never disclosed to an attacker.

   Care should be taken before forwarding a user's Kerberos credentials
   to the remote server.  If the remote server is not trustworthy, this
   could result in the user's credentials being compromised.  Hence, the
   user interface should not forward credentials by default; it would be
   far safer to either require the user to explicitly request
   credentials forwarding for each connection, or to have a trusted list
   of hosts for which credentials forwarding is enabled, but to not
   enable credentials forwarding by default for all machines.

   This mechanism does not include all of the telnet authentication
   negotiation exchanges in the integrity checksum as recommended in [2].
   This means that the selection of this option is vulnerable to
   downgrade attacks when multiple authentication type pairs are offered
   by the server.

   The IAC SB AUTHENTICATION NAME name IAC SE message is unprotected
   in the AUTH option and it is not verified by the AUTH KRB5 suboption.
   The name MUST be verified by a secure method after authentication
   completes before it is used to access authorization information or
   perform a login.  One method of verification is for the server to
   request the client to transmit the desired name using the
   TELNET NEW-ENVIRONMENT option [6].  This can be accomplished by
   sending the message after authentication and encryption activation
   completes:
     IAC SB NEW-ENVIRONMENT SEND VAR USER IAC SE
   The client's response should include the same name that was
   transmitted as part of the IAC SB AUTHENTICATION NAME name IAC SE
   message.


7. IANA Considerations

   The authentication type KERBEROS_V5 and its associated suboption
   values are registered with IANA.  Any suboption values used to extend
   the protocol as described in this document must be registered with
   IANA before use.  IANA is instructed not to issue new suboption
   values without submission of documentation of their use.


8. Normative References

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

   [RFC 4120] Neuman, C., T. Yu, and K. Raeburn, "The Kerberos Network
       Authentication System (V5)", RFC 4120, July 2005.

   [RFC YYYY] J. Altman, "Telnet Authentication Option",
       draft-altman-telnet-rfc2941bis-??.txt.

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

   [RFC 855] Postel, J. and J. Reynolds, "Telnet Option Specifications",
       STD 8, RFC 855, May 1983.

   [RFC XXXX] Altman, J., "Telnet START_TLS Option",
       draft-altman-telnet-starttls-??.txt.

   [RFC 1572] "Telnet (New) Environment Option", RFC 1572

   [RFC 2119] Bradner, S. "Key words for use in RFCs to Indicate
       Requirement Levels", RFC2119, March 1997.

   [RFC 4346] Dierks, T. and E. Rescorla. "The Transport Layer Security
       (TLS) Protocol Version 1.1.", RFC4346, April 2006.


9. Acknowledgments

   This document was originally written by Dave Borman of Cray Research,
   Inc.  Theodore Ts'o of MIT revised it to reflect the latest
   implementation experience.  Cliff Neuman and Prasad Upasani of USC's
   Information Sciences Institute developed the credential forwarding
   support.

   In addition, the contributions of the Telnet Working Group are also
   gratefully acknowledged.


10. Editor's Address

   Jeffrey Altman
   Secure Endpoints Inc
   255 W 94th Street
   New York NY 10025

   EMail: jaltman@secure-endpoints.com


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