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Versions: 00                                                            
Network Working Group                                  P. Overell
Internet Draft: ROAMING-ELGAMAL                Demon Internet Ltd
Document: draft-overell-roaming-elgamal-sasl-00.txt February 1998
                                             Expires: August 1998


           ROAMING-ELGAMAL SASL Authentication Mechanism

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
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   (Coast), or ftp.isi.edu (US West Coast).

Abstract

   ROAMING-ELGAMAL is an SASL [SASL] authentication mechanism in which
   ElGamal [ELG] public key cryptography is used to encrypt the persona
   and password thus giving a high degree of security.

   Although specifically designed for the Simple Roaming Authentication
   Protocol [SRAP], ROAMING-ELGAMAL is intended to be a registered SASL
   mechanism and so could be adapted to other protocols.  The mechanism
   has been designed to resist attack from interception, man in the
   middle, and replay.  The security of the mechanism rests with the
   protection of the private key.

1. Conventions Used in this Document

   In SASL terminology "server" means the authenticator and "client"
   means the authenticatee.  Data from the server to the client is a
   "challenge", data from the client to the server is a "response".

   All syntax is specified using ABNF [ABNF] and its core definitions.





Overell                                                  [Page 1]


Internet Draft           ROAMING-ELGAMAL            February 1998




2. ROAMING-ELGAMAL Mechanism

   The SASL authentication type associated with ROAMING-ELGAMAL is
   "ROAMING-ELGAMAL".

   This memo is only concerned with authentication, however, a security
   layer could be easily added either by continuing to use ElGamal
   encryption for the remainder of the conversation; or by using a
   symmetric cipher with a session key derived from the calculated
   value of y^k, a value known to both parties only after
   authentication is complete.

2.1 Initial Server Challenge

   The data encoded in the initial challenge is a persona, a
   fingerprint and a cookie.

   The persona indicates which persona/password pair the server is
   seeking authentication for.  If no persona is specified (zero size)
   then the client may choose either to return any of its
   persona/passwords or fail the command.  If the persona is specified
   but not recognized then the client SHOULD fail the command.  An
   implementation MAY choose the persona to be the same as a "username"
   or "user id" but this memo does not require this interpretation.

   The fingerprint indicates which public key the client should use to
   encrypt its response.  The fingerprint MAY be the MD5 or SHA-1 of
   the public key as per PGP, but this memo does not require this
   interpretation.

   The cookie is a presumptively arbitrary string of random octets.
   The cookie should be unique and unpredictable, preferably a
   cryptographically strong random number.  Its purpose is to defeat
   replay attack.

   This memo does not define the length or content of the persona,
   fingerprint or cookie.  To permit any octet to be used each element
   is preceded by its size in octets expressed as a number in text
   enclosed in braces.  The encoding used is defined by the host
   protocol.

   Syntax

      unencoded-challenge = size persona size fingerprint size cookie

      persona = *OCTET

      fingerprint = *OCTET




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Internet Draft           ROAMING-ELGAMAL            February 1998




      cookie = *OCTET

      size = "{" 1*DIGIT "}"

   Example (fictitious)

      {4}paul{32}AB246508F5217B54C77D3400239BCA45{16}9723763476348973

2.2 Client's Response

   If the client wishes to proceed then it responds with an encoded
   string of the ElGamal encrypted string of the PKCS#1 [PKCS#1] packed
   string consisting of the client's persona, password and the server's
   cookie.  The server's public key is used for the encryption.  This
   memo does not describe how the client may obtain the server's public
   key.  The encoding used is defined by the host protocol

   This memo places no restriction whatsoever on the content or length
   of persona, password or cookie.  In the unpacked-response each
   element is preceded by its size in octets expressed as a number in
   text enclosed in braces.

   Syntax

      persona = *OCTET

      password = *OCTET

      cookie = *OCTET

      size = "{" 1*DIGIT "}"

      unpacked-response = size persona size password size cookie

      packed-response = %x00 %x02 8*padding %x00 unpacked-response

      padding = %x01-FF

   Example (fictitious)

      {4}paul{8}sausages{20}b_basdlwyweyfbc73m8f

2.2.1 Packing and Encryption

   Let L be the length in octets of the ElGamal encryption modulus.

   If the length of the unpacked-response is greater than L - 11 octets
   then the unpacked-response is split into sections, each of which
   must be less than or equal to L - 11 octets long.



Overell                                                  [Page 3]


Internet Draft           ROAMING-ELGAMAL            February 1998




   Each unpacked-response-section is then packed according to [PKCS#1]
   by preceding the unpacked-response-section with octet 0, octet 2, a
   padding string, and an octet 0.  The padding string consists of at
   least eight non-zero random octets.  The total length of the packed
   form is the same as the length of the ElGamal encryption modulus.

   To encrypt a packed-response-section

   Given the client's public key (p, g, y) where p is the prime modulus
   and y = g^x mod p where x is the private key.

   M is the packed-response-section considered to be an integer with
   the first octet being the most significant.

   Pick a random number k

   a = g^k mod p

   b = y^k M mod p

   The encrypted-response-section is ab.  These two numbers are
   expressed as string consisting of two multiprecision fields as
   defined in [PGPFormat].

      Definition.  A multiprecision field is the concatenation of two
      fields:

         (a) a whole number field of length 2, with value B;
         (b) a whole number field, with value V.

      Field (b) is of length [(B+7)/8], i.e., the greatest integer
      which is no larger than (B+7)/8.  The value of the
      multiprecision field is defined to be V.  V must be between
      2^{B-1} and 2^B - 1 inclusive.  In other words B must be exactly
      the number of significant bits in V.

   The encrypted-response is formed by concatenating all of the
   encrypted-response-sections.

   The encrypted-response is then encoded according to the host
   protocol.

2.3 Authentication

   The server then decodes, decrypts and unpacks the string and then
   verifies the persona, password and cookie.  If correct the client is
   deemed to be authenticated.





Overell                                                  [Page 4]


Internet Draft           ROAMING-ELGAMAL            February 1998




   ElGamal decryption is given by

   M = b/a^x mod p

3. References

   [ABNF]   RFC2234, "Augmented BNF for syntax specifications: ABNF",
            D. Crocker and P. Overell, November 1997.

   [ELG]    "A Public-Key Cryptosystem and a Signature Scheme Based
            on Discrete Logarithms".  T. ElGamal, IEEE Transactions
            on Information Theory, v. IT-31, n. 4, 1985, pp. 469-472.

   [PGPForm] RFC1991, "PGP Message Exchange Formats".  D. Atkins et
            al.  August 1996.

   [PKCS#1] RSA Data Security, Inc.  Public-Key Cryptography
            Standards (PKCS).  PKCS #1, "RSA Encryption Standard".
            An RSA Laboratories Technical Note, version 1.5, revised
            November 1, 1993.

   [SASL]   RFC2222, "Simple Authentication and Security Layer
            (SASL)".  J. Myers, Netscape Communications, October
            1997.

   [SRAP]   Work in progress, "Simple Roaming Authentication
            Protocol", P. Overell, Demon Internet Ltd.  February 1998

4. Security Considerations

   The use of ElGamal public key encryption together with a
   cryptographically strong cookie should make this mechanism resistant
   to interception, man in the middle and replay attacks.

5. Author's Address

   P. Overell
   Demon Internet Ltd
   Dorking Business Park
   Dorking
   Surrey
   RH4 1HN
   UK

   mailto:paulo@turnpike.com







Overell                                                  [Page 5]