ecc-pm1-redc.c

   1 /* ecc-pm1-redc.c
   2
   3    Copyright (C) 2013, 2014 Niels Möller
   4
   5    This file is part of GNU Nettle.
   6
   7    GNU Nettle is free software: you can redistribute it and/or
   8    modify it under the terms of either:
   9
  10      * the GNU Lesser General Public License as published by the Free
  11        Software Foundation; either version 3 of the License, or (at your
  12        option) any later version.
  13
  14    or
  15
  16      * the GNU General Public License as published by the Free
  17        Software Foundation; either version 2 of the License, or (at your
  18        option) any later version.
  19
  20    or both in parallel, as here.
  21
  22    GNU Nettle is distributed in the hope that it will be useful,
  23    but WITHOUT ANY WARRANTY; without even the implied warranty of
  24    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  25    General Public License for more details.
  26
  27    You should have received copies of the GNU General Public License and
  28    the GNU Lesser General Public License along with this program.  If
  29    not, see http://www.gnu.org/licenses/.
  30 */
  31
  32 /* Development of Nettle's ECC support was funded by the .SE Internet Fund. */
  33
  34 #if HAVE_CONFIG_H
  35 # include "config.h"
  36 #endif
  37
  38 #include <assert.h>
  39
  40 #include "ecc-internal.h"
  41
  42 /* Use that 1 = - (p - 1) (mod p), and that at least one low limb of p
  43    - 1 is zero. */
  44 void
  45 ecc_pm1_redc (const struct ecc_modulo *m, mp_limb_t *rp, mp_limb_t *xp)
  46 {
  47   unsigned i;
  48   mp_limb_t hi, cy;
  49   unsigned shift = m->size * GMP_NUMB_BITS - m->bit_size;
  50   mp_size_t k = m->redc_size;
  51
  52   for (i = 0; i < m->size; i++)
  53     xp[i] = mpn_submul_1 (xp + i + k,
  54                           m->redc_mpm1, m->size - k, xp[i]);
  55   hi = mpn_sub_n (xp, xp + m->size, xp, m->size);
  56   cy = mpn_cnd_add_n (hi, rp, xp, m->m, m->size);
  57   assert_maybe (cy == hi);
  58
  59   if (shift > 0)
  60     {
  61       /* Result is always < 2p, provided that
  62          2^shift * Bmodp_shifted <= p */
  63       hi = (rp[m->size - 1] >> (GMP_NUMB_BITS - shift));
  64       rp[m->size - 1] = (rp[m->size - 1]
  65                            & (((mp_limb_t) 1 << (GMP_NUMB_BITS - shift)) - 1))
  66         + mpn_addmul_1 (rp, m->B_shifted, m->size-1, hi);
  67     }
  68 }