Secure Telnet Working Group                    Russell Housley (SPYRUS)
                                                  Todd Horting (SPYRUS)
Internet-Draft                                       Peter Yee (SPYRUS)
Expire in six months                                      February 1998


                    Telnet Authentication Using DSA

                 <draft-housley-telnet-auth-dsa-00.txt>


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are Draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ''work in progress.''

   To learn the current status of any Internet-Draft, please check the
   "1id-abstRacts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

   Distribution of this memo is unlimited.  Please send comments to the
   <telnet-ietf@bsdi.com> mailing list.


Abstract

   This document defines a method to authenticate telnet using the
   Digital Signature Algorithm (DSA) [2].  It relies on the Telnet
   Authentication Option [1].


1 Introduction

   The Telnet protocol provides no protocol security. Telnet servers may
   require users to login.  This is typically a host level login
   consisting of a user name and a password, transmitted in the clear.

   The mechanism specified in this document relies on the Telnet
   Authentication Option [1].




Housley, Horting & Yee                                          [Page 1]


INTERNET DRAFT                                             February 1998


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 normally 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 provide 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 these options, the
   parties are then able to perform subnegotiations to determine 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
   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)

   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 [3] is used with DSA [2].

   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 [4], Standard for Public Key
   Cryptographic Entity Authentication Mechanisms, draft of 28 February
   1995, including Appendix A on ASN.1 encoding of messages and tokens.



Housley, Horting & Yee                                          [Page 2]


INTERNET DRAFT                                             February 1998


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.  Therefore, the exchanges below do not show the FIPS PUB
   196 notations indicating optional fields, as all subfields used are
   mandatory.  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).  In
   Figure 1, the client binds the last transmission (token identifier,
   certificate, and token) together as an ASN.1 SEQUENCE.

   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 DSA authentication, the two-octet authentication type
   pair is "DSA CLIENT-TO-SERVER ONE-WAY ENCRYPT_OFF INI_CRED_FWD_OFF."
   This is encoded as two-octets: '0B01' in hexadecimal.  This indicates
   that the DSA authentication mechanism will be used to authenticate
   the client to the server, and that no encryption will be performed.

   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 suboption identifier is
   carried immediately after the two-octet authentication type pair.

   The exchanges detailed below, in Figure 1, 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, Horting & Yee                                          [Page 3]


INTERNET DRAFT                                             February 1998


   ---------------------------------------------------------------------
    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
    '0B01' '1' IAC SE              -->

                                       <-- IAC SB AUTHENTICATION REPLY
                                           '0B01' '2'
                                           Base64( Sequence( TokenID,
                                                               TokenBA ) )
                                           IAC SE

    IAC SB AUTHENTICATION IS
    '0B01' '3'
    Base64( Sequence(
        TokenID, CertA, TokenAB ) )
    IAC SE                          -->
   ---------------------------------------------------------------------
                                 Figure 1

3.2  Mutual Authentication with DSA

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

   For mutual DSA authentication, the two-octet authentication type pair
   is "DSA CLIENT-TO-SERVER MUTUAL ENCRYPT_OFF INI_CRED_FWD_OFF."  This
   is encoded as two-octets: '0B03' in hexadecimal.  This indicates that
   the DSA authentication mechanism will be used to mutually
   authenticate the client and the server, and that no encryption will
   be performed.









Housley, Horting & Yee                                          [Page 4]


INTERNET DRAFT                                             February 1998


   ---------------------------------------------------------------------
    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
    '0B03' '1' IAC SE             -->

                                       <-- IAC SB AUTHENTICATION REPLY
                                           '0B03' '2'
                                           Base64( Sequence( TokenID,
                                                               TokenBA ) )
                                           IAC SE

    IAC SB AUTHENTICATION IS
    '0B03' '3'
    Base64( Sequence(
      TokenID, CertA, TokenAB ) )
    IAC SE                        -->

                                       <-- IAC SB AUTHENTICATION REPLY
                                           '0B03' '4'
                                           Base64( Sequence( TokenID,
                                                       CertB, TokenBA2 ) )
                                           IAC SE
   ---------------------------------------------------------------------
                                 Figure 2

4.0  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.

5.0  Acknowledgements

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




Housley, Horting & Yee                                          [Page 5]


INTERNET DRAFT                                             February 1998


6.0  References

   [1] - Borman, David A. "Telnet Authentication Option".
         RFC 1416.  February 1993.

   [2] - Digital Signature Standard (DSS). FIPS Pub 186.
         May 19, 1994.

   [3] - Secure Hash Standard. FIPS Pub 180-1. April 17, 1995.

   [4] - Standard for Entity Authentication Using Public Key
         Cryptography.  FIPS Pub 196. February 18, 1997.

7.0  Author's Address

   Russell Housley
   SPYRUS
   PO Box 1198
   Herndon, VA 20172
   USA

   Phone: +1 703 435-7344
   Email: housley@spyrus.com


   Todd Horting
   SPYRUS
   PO Box 1198
   Herndon, VA 20172
   USA

   Phone: +1 703 435-4711
   Email: thorthing@spyrus.com


   Peter Yee
   SPYRUS
   2460 N. First Street
   Suite 100
   San Jose, CA 95131-1023
   USA

   Phone: +1 408 432-8180
   Email: yee@spyrus.com







Housley, Horting & Yee                                          [Page 6]