rfc2943.txt

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Network Working Group                                         R. Housley
Request for Comments: 2943                                    T. Horting
Category: Standards Track                                         P. Yee
                                                                  SPYRUS
                                                          September 2000


                    TELNET Authentication Using DSA

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 defines a telnet authentication mechanism using the
   Digital Signature Algorithm (DSA) [FIPS186].  It relies on the Telnet
   Authentication Option [RFC2941].

1. Command Names and Codes

    AUTHENTICATION           37

      Authentication Commands:

        IS                       0
        SEND                     1
        REPLY                    2
        NAME                     3

      Authentication Types:

        DSS                     14

      Modifiers:

        AUTH_WHO_MASK            1
        AUTH_CLIENT_TO_SERVER    0
        AUTH_SERVER_TO CLIENT    1




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RFC 2943            TELNET Authentication Using DSA       September 2000


        AUTH_HOW_MASK            2
        AUTH_HOW_ONE_WAY         0
        AUTH_HOW_MUTUAL          2

        ENCRYPT_MASK            20
        ENCRYPT_OFF              0
        ENCRYPT_USING_TELOPT     4
        ENCRYPT_AFTER_EXCHANGE  16
        ENCRYPT_RESERVED        20

        INI_CRED_FWD_MASK        8
        INI_CRED_FWD_OFF         0
        INI_CRED_FWD_ON          8

      Sub-option Commands:

        DSS_INITIALIZE           1
        DSS_TOKENBA              2
        DSS_CERTA_TOKENAB        3
        DSS_CERTB_TOKENBA2       4

2. TELNET Security Extensions

   TELNET, as a protocol, has no concept of security.  Without
   negotiated options, it merely passes characters back and forth
   between the NVTs represented by the two TELNET processes.  In its
   most common usage as a protocol for remote terminal access (TCP port
   23), TELNET connects to a server that requires user-level
   authentication through a user name and password in the clear; the
   server does not authenticate itself to the user.

   The TELNET Authentication Option provides for user authentication and
   server authentication.  User authentication replaces or augments the
   normal host password mechanism.  Server authentication is normally
   done in conjunction with user authentication.

   In order to support these security services, the two TELNET entities
   must first negotiate their willingness to support the TELNET
   Authentication Option.  Upon agreeing to support this option, the
   parties are then able to perform sub-option negotiations to the
   authentication protocol to be used, and possibly the remote user name
   to be used for authorization checking.

   Authentication and parameter negotiation occur within an unbounded
   series of exchanges.  The server proposes a preference-ordered list
   of authentication types (mechanisms) which it supports.  In addition
   to listing the mechanisms it supports, the server qualifies each
   mechanism with a modifier that specifies whether the authentication



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RFC 2943            TELNET Authentication Using DSA       September 2000


   is to be one-way or mutual, and in which direction the authentication
   is to be performed.  The client selects one mechanism from the list
   and responds to the server indicating its choice and the first set of
   authentication data needed for the selected authentication type.  The
   server and the client then proceed through whatever number of
   iterations are required to arrive at the requested authentication.

3. Use of Digital Signature Algorithm (DSA)

   DSA is also known as the Digital Signature Standard (DSS), and the
   names are used interchangeably.  This paper specifies a method in
   which DSA may be used to achieve certain security services when used
   in conjunction with the TELNET Authentication Option.  SHA-1
   [FIPS180-1] is used with DSA [FIPS186].

   DSA may provide either unilateral or mutual authentication.  Due to
   TELNET's character-by-character nature, it is not well-suited to the
   application of integrity-only services, therefore use of the DSA
   profile provides authentication but it does not provide session
   integrity.  This specification follows the token and exchanges
   defined in NIST FIPS PUB 196 [FIPS196], Standard for Public Key
   Cryptographic Entity Authentication Mechanisms including Appendix A
   on ASN.1 encoding of messages and tokens.  All data that is covered
   by a digital signature must be encoded using the Distinguished
   Encoding Rules (DER).  However, other data may use either the Basic
   Encoding Rules (BER) or DER [X.208].

3.1.  Unilateral Authentication with DSA

   Unilateral authentication must be done client-to-server.  What
   follows are the protocol steps necessary to perform DSA
   authentication as specified in FIPS PUB 196 under the TELNET
   Authentication Option framework.  Where failure modes are
   encountered, the return codes follow those specified in the TELNET
   Authentication Option.  They are not enumerated here, as they are
   invariant among the mechanisms used.  FIPS PUB 196 employs a set of
   exchanges that are transferred to provide authentication.  Each
   exchange employs various fields and tokens, some of which are
   optional.  In addition, each token has several subfields that are
   optional.  A conformant subset of the fields and subfields have been
   selected.  The tokens are ASN.1 encoded as defined in Appendix A of
   FIPS PUB 196, and each token is named to indicate the direction in
   which it flows (e.g., TokenBA flows from Party B to Party A).  All
   data that is covered by a digital signature must be encoded using the







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RFC 2943            TELNET Authentication Using DSA       September 2000


   Distinguished Encoding Rules (DER).  Data that is not covered by a
   digital signature may use either the Basic Encoding Rules (BER) or
   DER [X.208].  Figure 1 illustrates the exchanges for unilateral
   authentication.

   During authentication, the client may provide the user name to the
   server by using the authentication name sub-option.  If the name
   sub-option is not used, the server will generally prompt for a name
   and password in the clear.  The name sub-option must be sent after
   the server sends the list of authentication types supported and
   before the client finishes the authentication exchange, this ensures
   that the server will not prompt for a user name and password.  In
   figure 1, the name sub-option is sent immediately after the server
   presents the list of authentication types supported.

   For one-way DSS authentication, the two-octet authentication type
   pair is DSS AUTH_CLIENT_TO_SERVER | AUTH_HOW_ONE_WAY | ENCRYPT_OFF |
   INI_CRED_FWD_OFF.  This indicates that the DSS authentication
   mechanism will be used to authenticate the client to the server and
   that no encryption will be performed.

   CertA is the clients certificate.  Both certificates are X.509
   certificates that contain DSS public keys[RFC2459].  The client must
   validate the server's certificate before using the DSA public key it
   contains.

   Within the unbounded authentication exchange, implementation is
   greatly simplified if each portion of the exchange carries a unique
   identifier.  For this reason, a single octet sub-option identifier is
   carried immediately after the two-octet authentication type pair.

   The exchanges detailed in Figure 1 below presume knowledge of FIPS
   PUB 196 and the TELNET Authentication Option.  The client is Party A,
   while the server is Party B.  At the end of the exchanges, the client
   is authenticated to the server.
















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RFC 2943            TELNET Authentication Using DSA       September 2000


------------------------------------------------------------------
 Client (Party A)                   Server (Party B)

                                 <-- IAC DO AUTHENTICATION

 IAC WILL AUTHENTICATION     -->

                                 <-- IAC SB AUTHENTICATION SEND
                                     <list of authentication options>
                                     IAC SE

 IAC SB AUTHENTICATION
 NAME <user name>            -->

 IAC SB AUTHENTICATION IS
 DSS
 AUTH_CLIENT_TO_SERVER |
     AUTH_HOW_ONE_WAY |
     ENCRYPT_OFF |
     INI_CRED_FWD_OFF
 DSS_INITIALIZE
 IAC SE                     -->

                                 <-- IAC SB AUTHENTICATION REPLY
                                     DSS
                                     AUTH_CLIENT_TO_SERVER |
                                         AUTH_HOW_ONE_WAY |
                                         ENCRYPT_OFF |
                                         INI_CRED_FWD_OFF
                                     DSS_TOKENBA
                                     Sequence( TokenID, TokenBA )
                                     IAC SE

 IAC SB AUTHENTICATION IS
 DSS
 AUTH_CLIENT_TO_SERVER |
     AUTH_HOW_ONE_WAY |
     ENCRYPT_OFF |
     INI_CRED_FWD_OFF
 DSS_CERTA_TOKENAB
 Sequence( TokenID, CertA, TokenAB )
 IAC SE                     -->
------------------------------------------------------------------
                              Figure 1







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RFC 2943            TELNET Authentication Using DSA       September 2000


3.2.  Mutual Authentication with DSA

   Mutual authentication is slightly more complex.  Figure 2 illustrates
   the exchanges.

   For mutual DSS authentication, the two-octet authentication type pair
   is DSS AUTH_CLIENT_TO_SERVER | AUTH_HOW_MUTUAL | ENCRYPT_OFF |
   INI_CRED_FWD_OFF.  This indicates that the DSS authentication
   mechanism will be used to mutually authenticate the client and the
   server and that no encryption will be performed.

---------------------------------------------------------------------
 Client (Party A)                   Server (Party B)

IAC WILL AUTHENTICATION        -->

                                    <-- IAC DO AUTHENTICATION

                                    <-- IAC SB AUTHENTICATION SEND
                                        <list of authentication options>
                                        IAC SE

 IAC SB AUTHENTICATION
 NAME <user name>              -->

 IAC SB AUTHENTICATION IS
 DSS
 AUTH_CLIENT_TO_SERVER |
     AUTH_HOW_MUTUAL |
     ENCRYPT_OFF |
     INI_CRED_FWD_OFF
 DSS_INITIALIZE
 IAC SE                        -->

                                    <-- IAC SB AUTHENTICATION REPLY
                                        DSS
                                        AUTH_CLIENT_TO_SERVER |
                                            AUTH_HOW_MUTUAL |
                                            ENCRYPT_OFF |
                                            INI_CRED_FWD_OFF
                                        DSS_TOKENBA
                                        Sequence( TokenID, TokenBA )
                                        IAC SE








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