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/* dsa-verify.c
*
* The DSA publickey algorithm.
*/
/* nettle, low-level cryptographics library
*
* Copyright (C) 2002, 2003 Niels Möller
*
* The nettle library is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2.1 of the License, or (at your
* option) any later version.
*
* The nettle library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
* License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with the nettle library; see the file COPYING.LIB. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
* MA 02111-1301, USA.
*/
#if HAVE_CONFIG_H
# include "config.h"
#endif
#include <stdlib.h>
#include "dsa.h"
#include "bignum.h"
int
_dsa_verify(const struct dsa_public_key *key,
unsigned digest_size,
const uint8_t *digest,
const struct dsa_signature *signature)
{
mpz_t w;
mpz_t tmp;
mpz_t v;
int res;
if (mpz_sizeinbase(key->q, 2) != 8 * digest_size)
return 0;
/* Check that r and s are in the proper range */
if (mpz_sgn(signature->r) <= 0 || mpz_cmp(signature->r, key->q) >= 0)
return 0;
if (mpz_sgn(signature->s) <= 0 || mpz_cmp(signature->s, key->q) >= 0)
return 0;
mpz_init(w);
/* Compute w = s^-1 (mod q) */
/* NOTE: In gmp-2, mpz_invert sometimes generates negative inverses,
* so we need gmp-3 or better. */
if (!mpz_invert(w, signature->s, key->q))
{
mpz_clear(w);
return 0;
}
mpz_init(tmp);
mpz_init(v);
/* The message digest */
nettle_mpz_set_str_256_u(tmp, digest_size, digest);
/* v = g^{w * h (mod q)} (mod p) */
mpz_mul(tmp, tmp, w);
mpz_fdiv_r(tmp, tmp, key->q);
mpz_powm(v, key->g, tmp, key->p);
/* y^{w * r (mod q) } (mod p) */
mpz_mul(tmp, signature->r, w);
mpz_fdiv_r(tmp, tmp, key->q);
mpz_powm(tmp, key->y, tmp, key->p);
/* v = (g^{w * h} * y^{w * r} (mod p) ) (mod q) */
mpz_mul(v, v, tmp);
mpz_fdiv_r(v, v, key->p);
mpz_fdiv_r(v, v, key->q);
res = !mpz_cmp(v, signature->r);
mpz_clear(w);
mpz_clear(tmp);
mpz_clear(v);
return res;
}
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