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/* ecc-generic-redc.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 Internetfonden. */
#if HAVE_CONFIG_H
# include "config.h"
#endif
#include <assert.h>
#include "ecc-internal.h"
void
ecc_generic_redc (const struct ecc_curve *ecc, mp_limb_t *rp)
{
unsigned i;
mp_limb_t hi, cy;
unsigned shift = ecc->size * GMP_NUMB_BITS - ecc->bit_size;
mp_size_t k = ecc->redc_size;
assert (k != 0);
if (k > 0)
{
/* Use that 1 = p + 1, and that at least one low limb of p + 1 is zero. */
for (i = 0; i < ecc->size; i++)
rp[i] = mpn_addmul_1 (rp + i + k,
ecc->redc_ppm1, ecc->size - k, rp[i]);
hi = mpn_add_n (rp, rp, rp + ecc->size, ecc->size);
if (shift > 0)
{
hi = (hi << shift) | (rp[ecc->size - 1] >> (GMP_NUMB_BITS - shift));
rp[ecc->size - 1] = (rp[ecc->size - 1]
& (((mp_limb_t) 1 << (GMP_NUMB_BITS - shift)) - 1))
+ mpn_addmul_1 (rp, ecc->Bmodp_shifted, ecc->size-1, hi);
}
else
{
cy = cnd_sub_n (hi, rp, ecc->p, ecc->size);
assert (cy == hi);
}
}
else
{
/* Use that 1 = - (p - 1), and that at least one low limb of p -
1 is zero. */
k = -k;
for (i = 0; i < ecc->size; i++)
rp[i] = mpn_submul_1 (rp + i + k,
ecc->redc_ppm1, ecc->size - k, rp[i]);
hi = mpn_sub_n (rp, rp + ecc->size, rp, ecc->size);
cy = cnd_add_n (hi, rp, ecc->p, ecc->size);
assert (cy == hi);
if (shift > 0)
{
/* Result is always < 2p, provided that
2^shift * Bmodp_shifted <= p */
hi = (rp[ecc->size - 1] >> (GMP_NUMB_BITS - shift));
rp[ecc->size - 1] = (rp[ecc->size - 1]
& (((mp_limb_t) 1 << (GMP_NUMB_BITS - shift)) - 1))
+ mpn_addmul_1 (rp, ecc->Bmodp_shifted, ecc->size-1, hi);
}
}
}
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