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* elgamal.c (gen_k): New arg SMALL_K. (sign): Use it here with SMALL_K set

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to false (do_encrypt): and here with SMALL_K set to true.  From Werner on
devel branch.
This commit is contained in:
David Shaw 2003-11-29 20:07:39 +00:00
parent 60cdcf61f5
commit dfd8b9d8d3
2 changed files with 39 additions and 27 deletions
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7
cipher/ChangeLog
View File

@ -1,3 +1,10 @@
2003-11-29 David Shaw <dshaw@jabberwocky.com>
* elgamal.c (gen_k): New arg SMALL_K.
(sign): Use it here with SMALL_K set to false
(do_encrypt): and here with SMALL_K set to true. From Werner on
devel branch.
2003-10-10 Werner Koch <wk@gnupg.org>
* primegen.c (gen_prime): Bail out if we try to generate a prime

59
cipher/elgamal.c
View File

@ -1,5 +1,5 @@
/* elgamal.c - ElGamal Public Key encryption
* Copyright (C) 1998, 2000, 2001 Free Software Foundation, Inc.
/* elgamal.c - elgamal Public Key encryption
* Copyright (C) 1998, 2000, 2001, 2003 Free Software Foundation, Inc.
*
* For a description of the algorithm, see:
* Bruce Schneier: Applied Cryptography. John Wiley & Sons, 1996.
@ -47,7 +47,7 @@ typedef struct {
static void test_keys( ELG_secret_key *sk, unsigned nbits );
static MPI gen_k( MPI p );
static MPI gen_k( MPI p, int small_k );
static void generate( ELG_secret_key *sk, unsigned nbits, MPI **factors );
static int check_secret_key( ELG_secret_key *sk );
static void do_encrypt(MPI a, MPI b, MPI input, ELG_public_key *pkey );
@ -139,11 +139,11 @@ test_keys( ELG_secret_key *sk, unsigned nbits )
do_encrypt( out1_a, out1_b, test, &pk );
decrypt( out2, out1_a, out1_b, sk );
if( mpi_cmp( test, out2 ) )
log_fatal("ElGamal operation: encrypt, decrypt failed\n");
log_fatal("Elgamal operation: encrypt, decrypt failed\n");
sign( out1_a, out1_b, test, sk );
if( !verify( out1_a, out1_b, test, &pk ) )
log_fatal("ElGamal operation: sign, verify failed\n");
log_fatal("Elgamal operation: sign, verify failed\n");
mpi_free( test );
mpi_free( out1_a );
@ -153,11 +153,12 @@ test_keys( ELG_secret_key *sk, unsigned nbits )
/****************
* generate a random secret exponent k from prime p, so
* that k is relatively prime to p-1
* Generate a random secret exponent k from prime p, so that k is
* relatively prime to p-1. With SMALL_K set, k will be selected for
* better encryption performance - this must never bee used signing!
*/
static MPI
gen_k( MPI p )
gen_k( MPI p, int small_k )
{
MPI k = mpi_alloc_secure( 0 );
MPI temp = mpi_alloc( mpi_get_nlimbs(p) );
@ -167,13 +168,18 @@ gen_k( MPI p )
unsigned int nbytes;
char *rndbuf = NULL;
/* IMO using a k much lesser than p is sufficient and it greatly
* improves the encryption performance. We use Wiener's table
* and add a large safety margin.
*/
nbits = wiener_map( orig_nbits ) * 3 / 2;
if( nbits >= orig_nbits )
BUG();
if (small_k)
{
/* Using a k much lesser than p is sufficient for encryption and
* it greatly improves the encryption performance. We use
* Wiener's table and add a large safety margin.
*/
nbits = wiener_map( orig_nbits ) * 3 / 2;
if( nbits >= orig_nbits )
BUG();
}
else
nbits = orig_nbits;
nbytes = (nbits+7)/8;
if( DBG_CIPHER )
@ -184,8 +190,8 @@ gen_k( MPI p )
m_free(rndbuf);
rndbuf = get_random_bits( nbits, 1, 1 );
}
else { /* change only some of the higher bits */
/* we could impprove this by directly requesting more memory
else { /* Change only some of the higher bits. */
/* We could impprove this by directly requesting more memory
* at the first call to get_random_bits() and use this the here
* maybe it is easier to do this directly in random.c
* Anyway, it is highly inlikely that we will ever reach this code
@ -193,14 +199,10 @@ gen_k( MPI p )
char *pp = get_random_bits( 32, 1, 1 );
memcpy( rndbuf,pp, 4 );
m_free(pp);
log_debug("gen_k: tsss, never expected to reach this\n");
}
mpi_set_buffer( k, rndbuf, nbytes, 0 );
for(;;) {
/* Hmm, actually we don't need this step here
* because we use k much smaller than p - we do it anyway
* just in case the keep on adding a one to k ;) */
if( !(mpi_cmp( k, p_1 ) < 0) ) { /* check: k < (p-1) */
if( DBG_CIPHER )
progress('+');
@ -262,10 +264,13 @@ generate( ELG_secret_key *sk, unsigned int nbits, MPI **ret_factors )
* secret part. The prime is public and may be shared anyway,
* so a random generator level of 1 is used for the prime.
*
* I don't see a reason to have a x of about the same size
* as the p. It should be sufficient to have one about the size
* of q or the later used k plus a large safety margin. Decryption
* will be much faster with such an x.
* I don't see a reason to have a x of about the same size as the
* p. It should be sufficient to have one about the size of q or
* the later used k plus a large safety margin. Decryption will be
* much faster with such an x. Note that this is not optimal for
* signing keys becuase it makes an attack using accidential small
* K values even easier. Well, one should not use ElGamal signing
* anyway.
*/
xbits = qbits * 3 / 2;
if( xbits >= nbits )
@ -347,7 +352,7 @@ do_encrypt(MPI a, MPI b, MPI input, ELG_public_key *pkey )
* error code.
*/
k = gen_k( pkey->p );
k = gen_k( pkey->p, 1 );
mpi_powm( a, pkey->g, k, pkey->p );
/* b = (y^k * input) mod p
* = ((y^k mod p) * (input mod p)) mod p
@ -413,7 +418,7 @@ sign(MPI a, MPI b, MPI input, ELG_secret_key *skey )
*
*/
mpi_sub_ui(p_1, p_1, 1);
k = gen_k( skey->p );
k = gen_k( skey->p, 0 /* no small K ! */ );
mpi_powm( a, skey->g, k, skey->p );
mpi_mul(t, skey->x, a );
mpi_subm(t, input, t, p_1 );