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/* dsa-sign.c
*
* The DSA publickey algorithm.
*/
/* nettle, low-level cryptographics library
*
* Copyright (C) 2002, 2010 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., 59 Temple Place - Suite 330, Boston,
* MA 02111-1307, USA.
*/
#if HAVE_CONFIG_H
# include "config.h"
#endif
#include <assert.h>
#include <stdlib.h>
#include "dsa.h"
#include "bignum.h"
int
_dsa_sign(const struct dsa_public_key *pub,
const struct dsa_private_key *key,
void *random_ctx, nettle_random_func random,
unsigned digest_size,
const uint8_t *digest,
struct dsa_signature *signature)
{
mpz_t k;
mpz_t h;
mpz_t tmp;
/* Require precise match of bitsize of q and hash size. The general
description of DSA in FIPS186-3 allows both larger and smaller q;
in the the latter case, the hash must be truncated to the right
number of bits. */
if (mpz_sizeinbase(pub->q, 2) != 8 * digest_size)
return 0;
/* Select k, 0<k<q, randomly */
mpz_init_set(tmp, pub->q);
mpz_sub_ui(tmp, tmp, 1);
mpz_init(k);
nettle_mpz_random(k, random_ctx, random, tmp);
mpz_add_ui(k, k, 1);
/* Compute r = (g^k (mod p)) (mod q) */
mpz_powm(tmp, pub->g, k, pub->p);
mpz_fdiv_r(signature->r, tmp, pub->q);
/* Compute hash */
mpz_init(h);
nettle_mpz_set_str_256_u(h, digest_size, digest);
/* Compute k^-1 (mod q) */
if (!mpz_invert(k, k, pub->q))
/* What do we do now? The key is invalid. */
return 0;
/* Compute signature s = k^-1 (h + xr) (mod q) */
mpz_mul(tmp, signature->r, key->x);
mpz_fdiv_r(tmp, tmp, pub->q);
mpz_add(tmp, tmp, h);
mpz_mul(tmp, tmp, k);
mpz_fdiv_r(signature->s, tmp, pub->q);
mpz_clear(k);
mpz_clear(h);
mpz_clear(tmp);
return 1;
}
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