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rfc:rfc2943

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

Housley, et al. Standards Track [Page 1] 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

Housley, et al. Standards Track [Page 2] 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

Housley, et al. Standards Track [Page 3] 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.

Housley, et al. Standards Track [Page 4] 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

Housley, et al. Standards Track [Page 5] 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

Housley, et al. Standards Track [Page 6] RFC 2943 TELNET Authentication Using DSA September 2000

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

Housley, et al. Standards Track [Page 7] RFC 2943 TELNET Authentication Using DSA September 2000

 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

Housley, et al. Standards Track [Page 8] RFC 2943 TELNET Authentication Using DSA September 2000

    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.

Housley, et al. Standards Track [Page 9] RFC 2943 TELNET Authentication Using DSA September 2000

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.

Housley, et al. Standards Track [Page 10] RFC 2943 TELNET Authentication Using DSA September 2000

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

Housley, et al. Standards Track [Page 11] RFC 2943 TELNET Authentication Using DSA September 2000

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.

Housley, et al. Standards Track [Page 12]

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