





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

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

                                    <-- IAC SB AUTHENTICATION REPLY
                                        DSS
                                        AUTH_CLIENT_TO_SERVER |
                                            AUTH_HOW_MUTUAL |
                                            ENCRYPT_OFF |
                                            INI_CRED_FWD_OFF
                                        DSS_CERTB_TOKENBA2
                                        Sequence( TokenID, CertB,
                                                  TokenBA2 )
                                        IAC SE
---------------------------------------------------------------------
                              Figure 2

4.  ASN.1 Syntax

   As stated earlier, a conformant subset of the defined fields and
   subfields from FIPS PUB 196 have been selected.  This section
   provides the ASN.1 syntax for that conformant subset.

   Figure 1 and Figure 2 include representations of the structures
   defined in this section.  Implementors should refer to the following
   table to determine the ASN.1 definitions that match the figure
   references:

      Figure 1   Sequence( TokenID, TokenBA )           MessageBA
                 Sequence( TokenID, CertA, TokenAB )    MessageAB

      Figure 2   Sequence( TokenID, TokenBA )           MessageBA
                 Sequence( TokenID, CertA, TokenAB )    MessageAB
                 Sequence( TokenID, CertB, TokenBA2 )   MessageBA2

   The following ASN.1 definitions specify the conformant subset of FIPS
   196.  For simplicity, no optional fields or subfields are included.
   The ASN.1 definition for CertificationPath is imported from CCITT
   Recommendation X.509 [X.509], and The ASN.1 definition for Name is
   imported from CCITT Recommendation X.501 [X.501].  These ASN.1



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   definitions are not repeated here.  All DSA signature values are
   encoded as a sequence of two integers, employing the same conventions
   specified in RFC 2459, section 7.2.2.

      MessageBA  ::=  SEQUENCE  {
        tokenId       [0] TokenId,
        tokenBA           TokenBA  }

      TokenBA  ::=  SEQUENCE  {
        ranB              RandomNumber,
        timestampB        TimeStamp  }

      MessageAB  ::=  SEQUENCE  {
        tokenId       [0] TokenId,
        certA         [1] CertData,
        tokenAB           TokenAB  }

      TokenAB  ::=  SEQUENCE  {
        ranA              RandomNumber,
        ranB              RandomNumber,
        entityB           EntityName,
        timestampB        TimeStamp,
        absigValue        OCTET STRING  }

      MessageBA2  ::=  SEQUENCE  {
        tokenId       [0] TokenId,
        certB         [1] CertData,
        tokenBA2          TokenBA2  }

      TokenBA2  ::=  SEQUENCE  {
        ranB          [0] RandomNumber,
        ranA          [1] RandomNumber,
        entityA           EntityName,
        timestampB2       TimeStamp,
        ba2sigValue       OCTET STRING  }

      CertData  ::=  SEQUENCE  {
        certPath      [0] CertificationPath  }  -- see X.509

      EntityName  ::=  SEQUENCE OF CHOICE  {    -- only allow one!
        directoryName [4] Name  }               -- see X.501

      RandomNumber  ::=  INTEGER                -- 20 octets








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      TokenId  ::=  SEQUENCE  {
        tokenType         INTEGER,              -- see table below
        protoVerNo        INTEGER  }            -- always 0x0001

      TimeStamp  ::=  GeneralizedTime

   The TokenId.TokenType is used to distinguish the message type and the
   authentication type (either unilateral or mutual).  The following
   table provides the values needed to implement this specification:

      Message Type    Authentication Type     TokenId.TokenType

        MessageBA       Unilateral              0x0001
                        Mutual                  0x0011

        MessageAB       Unilateral              0x0002
                        Mutual                  0x0012

        MessageBA       Mutual                  0x0013

5.  Security Considerations

   This entire memo is about security mechanisms.  For DSA to provide
   the authentication discussed, the implementation must protect the
   private key from disclosure.

   Implementations must randomly generate DSS private keys, 'k' values
   used in DSS signatures, and nonces.  The use of inadequate pseudo-
   random number generators (PRNGs) to generate cryptographic values can
   result in little or no security.  An attacker may find it much easier
   to reproduce the PRNG environment that produced the values, searching
   the resulting small set of possibilities, rather than using a brute
   force search.  The generation of quality random numbers is difficult.
   RFC 1750 [RFC1750] offers important guidance in this area, and
   Appendix 3 of FIPS PUB 186 [FIPS186] provides one quality PRNG
   technique.

6.  Acknowledgements

   We would like to thank William Nace for support during implementation
   of this specification.










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7. IANA Considerations

   The authentication type DSS 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.  References

   FIPS180-1 Secure Hash Standard. FIPS Pub 180-1. April 17, 1995.
             <http://csrc.nist.gov/fips/fips180-1.pdf>

   FIPS186   Digital Signature Standard (DSS). FIPS Pub 186.  May 19,
             1994. <http://csrc.nist.gov/fips/fips186.pdf>

   FIPS196   Standard for Entity Authentication Using Public Key
             Cryptography.  FIPS Pub 196. February 18, 1997.
             <http://csrc.nist.gov/fips/fips196.pdf>

   RFC1750   Eastlake, 3rd, D., Crocker, S. and J. Schiller, "Randomness
             Recommendations for Security", RFC 1750, December 1994.

   RFC2459   Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
             X.509 Public Key Infrastructure: X.509 Certificate and CRL
             Profile", RFC 2459, January 1999.

   RFC2941   T'so, T. and J. Altman, "Telnet Authentication Option", RFC
             2941, September 2000.

   X.208     CCITT.  Recommendation X.208: Specification of Abstract
             Syntax Notation One (ASN.1).  1988.

   X.501     CCITT. Recommendation X.501: The Directory - Models. 1988.

   X.509     CCITT.  Recommendation X.509: The Directory -
             Authentication Framework.  1988.














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9.  Authors' Addresses

   Russell Housley
   SPYRUS
   381 Elden Street, Suite 1120
   Herndon, VA 20172
   USA

   EMail: housley@spyrus.com


   Todd Horting
   SPYRUS
   381 Elden Street, Suite 1120
   Herndon, VA 20172
   USA

   EMail: thorting@spyrus.com


   Peter Yee
   SPYRUS
   5303 Betsy Ross Drive
   Santa Clara, CA 95054
   USA

   EMail: yee@spyrus.com
























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10.  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.



















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