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/* ecc-random.c */
/* nettle, low-level cryptographics library
*
* Copyright (C) 2013 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.
*/
/* Development of Nettle's ECC support was funded by the .SE Internet Fund. */
#if HAVE_CONFIG_H
# include "config.h"
#endif
#include <assert.h>
#include "ecc.h"
#include "ecc-internal.h"
#include "nettle-internal.h"
static int
zero_p (const struct ecc_curve *ecc,
const mp_limb_t *xp)
{
mp_limb_t t;
mp_size_t i;
for (i = t = 0; i < ecc->size; i++)
t |= xp[i];
return t == 0;
}
static int
ecdsa_in_range (const struct ecc_curve *ecc,
const mp_limb_t *xp, mp_limb_t *scratch)
{
/* Check if 0 < x < q, with data independent timing. */
return !zero_p (ecc, xp)
& (mpn_sub_n (scratch, xp, ecc->q, ecc->size) != 0);
}
void
ecc_modq_random (const struct ecc_curve *ecc, mp_limb_t *xp,
void *ctx, nettle_random_func *random, mp_limb_t *scratch)
{
uint8_t *buf = (uint8_t *) scratch;
unsigned nbytes = (ecc->bit_size + 7)/8;
/* The bytes ought to fit in the scratch area, unless we have very
unusual limb and byte sizes. */
assert (nbytes <= ecc->size * sizeof (mp_limb_t));
do
{
/* q and p are of the same bitsize. */
random (ctx, nbytes, buf);
buf[0] &= 0xff >> (nbytes * 8 - ecc->bit_size);
mpn_set_base256 (xp, ecc->size, buf, nbytes);
}
while (!ecdsa_in_range (ecc, xp, scratch));
}
void
ecc_scalar_random (struct ecc_scalar *x,
void *random_ctx, nettle_random_func *random)
{
TMP_DECL (scratch, mp_limb_t, ECC_MODQ_RANDOM_ITCH (ECC_MAX_SIZE));
TMP_ALLOC (scratch, ECC_MODQ_RANDOM_ITCH (x->ecc->size));
ecc_modq_random (x->ecc, x->p, random_ctx, random, scratch);
}
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