INTERNET DRAFT		EXPIRES FEB 1998	INTERNET DRAFT	
Internet Draft                                           Peter Gutmann
				                University of Auckland

 
 
                  ElGamal Profile for X.509 Certificates
                    <draft-rfced-info-pgutmann-00.txt>

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Abstract
 
This document describes the ASN.1 encoding for an X.509 certificate
profiled for use with the ElGamal public key cryptosystem [1].  It is
intended to provide guidelines for those developing software that will
be used to issue and use ElGamal certificates, and to ensure that
ElGamal certificate and key information will be handled consistently
throughout the public key infrastructure.
 
 
1. ASN.1 Definition of Certificate Elements
 
The abstract definition of X.509 certificates is given in [2].  The
elements specific to ElGamal are the algorithm identifier, the public
key information, and the signature data.  These are as follows:
 
-- ElGamal may be used in conjunction with the SHA-1 and RIPEMD-160
-- hash algorithms.  The ASN.1 object identifiers used to identify
--- the ElGamal signature algorithm when used with these two hash
-- algorithms is:
 
elGamalWithSHA-1 OBJECT IDENTIFIER ::= {
    {iso(1) org(3) dod(6) internet(1) private(4) enterprise(1)
dds(3029)
     signature(3) 1}
    }
 
elGamalWithRIPEMD-160 OBJECT IDENTIFIER :;= {
    {iso(1) org(3) dod(6) internet(1) private(4) enterprise(1)
dds(3029)
     signature(3) 2}
    }
 
-- Some of the ElGamal parameters may be shared among a number of
-- users.  These are conveyed in the parameters component of the
-- ElGamal AlgorithmIdentifier, and are as follows:
 
elGamal-Params ::= SEQUENCE {
    p           INTEGER,
    g           INTEGER
    }
 
-- The remaining ElGamal parameter is the users public key:
 
elGamalPublicKey ::= SEQUENCE {
    y           INTEGER,
    }
 
-- The AlgorithmIdentifier and public key are then encoded into a
-- standard X.509 SubjectPublicKeyInfo:
 
SubjectPublicKeyInfo ::= SEQUENCE {
    algorithm   AlgorithmIdentifier,
    subjectPublicKey    BIT STRING
    }
 
-- Prior to the bitstring encoding of an ElGamal signature, the
-- signature components are encoded  as follows:
 
elGamal-Sig ::= SEQUENCE {
    r           INTEGER,
    s           INTEGER
    }
 
 
2. Use of ElGamal for Encryption
 
The ElGamal algorithm may also be used for encryption.  In this case
the message formatting rules follow the rules for RSA encryption as
set
out in PKCS #1 [3], and use a message block type of 01.  The object
identifier for ElGamal encryption is:
 
elGamalEncryption OBJECT IDENTIFIER ::= {
    {iso(1) org(3) dod(6) internet(1) private(4) enterprise(1)
dds(3029)
     asymmetric-encryption(2) 1}
    }
 
The encrypted message consists of two components, the integers a = g^k
mod p and b = My^k mod p (this is not intended as an explanation of
the
ElGamal algorithm, but merely to indicate which integer is which).
The
encoding of these integers is:
 
elGamalEncryptedMessage ::= SEQUENCE {
    a           INTEGER,
    b           INTEGER
    }

Decryption follows the ElGamal algorithm, and the decrypted message is
again handled as per PKCS #1.
 
 
3. Security considerations
 
Although the use of the ElGamal algorithm for digital signature
generation is not directly addressed in this document, it should be
pointed out that some care needs to be taken with both the choice of
keys and the use of the algorithm.  Details on the safe use of ElGamal
are given in [4].  A weakness of ElGamal when used for digital
signatures, and workarounds to avoid the weakness, are given in [5].
 
Ongoing research into the security of ElGamal may reveal other factors
which need to be taken into account to provide adequate security for
signature and encryption applications, for example it is desirable
that
g generate a large subgroup of Zp*; it is recommended that
implementors
keep abreast of current research on the choice of parameters and use
of
the algorithm in order to avoid potential security weaknesses.
 
 
3. References
 
[1] "A public-key cryptosystem based on discrete logarithms", Taher
    ElGamal, IEEE Transactions on Information Theory, Vol.31, No.4
    (1985), p.469.
 
[2] ITU Recommendation X.509 (1993).
 
[3] Public-Key Cryptography Standard #1 (PKCS #1) v1.5, RSA Data
    Security Inc, November 1993.
 
[4] "Handbook of Applied Cryptography", Alfred Menezes, Paul van
    Oorschot, and Scott Vanstone, CRC Press, 1996.
 
[5] "Generating ElGamal signatures without knowing the secret key",
    Daniel Bleichenbacher, presented at EuroCrypt'96.
 
 
Author's Address
 
Peter Gutmann
University of Auckland
Private Bag 92019
Auckland
New Zealand
 
Phone: +64 9 373-7599
Email: pgut001@cs.auckland.ac.nz

INTERNET DRAFT		EXPIRES FEB 1998	INTERNET DRAFT