/* dsa-hash.c */
/* nettle, low-level cryptographics library
*
* Copyright (C) 2013 Niels MÃ¶ller
*
* 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 "dsa.h"
#include "bignum.h"
/* Convert hash value to an integer. 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. */
void
_dsa_hash (mpz_t h, unsigned bit_size,
size_t length, const uint8_t *digest)
{
if (length > (bit_size + 7) / 8)
length = (bit_size + 7) / 8;
nettle_mpz_set_str_256_u(h, length, digest);
if (8 * length > bit_size)
/* We got a few extra bits, at the low end. Discard them. */
mpz_tdiv_q_2exp (h, h, 8*length - bit_size);
}