ecc-mod.c 2.84 KB
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/* ecc-mod.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.
 */

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/* Development of Nettle's ECC support was funded by the .SE Internet Fund. */
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#if HAVE_CONFIG_H
# include "config.h"
#endif

#include <assert.h>

#include "ecc-internal.h"

/* Computes r mod m, where m is of size mn. bp holds B^mn mod m, as mn
   limbs, but the upper mn - bn libms are zero. */
void
ecc_mod (mp_limb_t *rp, mp_size_t rn, mp_size_t mn,
	 const mp_limb_t *bp, mp_size_t bn,
	 const mp_limb_t *b_shifted, unsigned shift)
{
  mp_limb_t hi;
  mp_size_t sn = mn - bn;
  mp_size_t i;

  assert (sn > 0);

  /* FIXME: Could use mpn_addmul_2. */
  /* Eliminate sn = mn - bn limbs at a time */
  if (bp[bn-1] < ((mp_limb_t) 1 << (GMP_NUMB_BITS - 1)))
    {
      /* Multiply sn + 1 limbs at a time, so we get a mn+1 limb
	 product. Then we can absorb the carry in the high limb */
      while (rn > 2 * mn - bn)
	{
	  rn -= sn;

	  for (i = 0; i <= sn; i++)
	    rp[rn+i-1] = mpn_addmul_1 (rp + rn - mn - 1 + i, bp, bn, rp[rn+i-1]);
	  rp[rn-1] = rp[rn+sn-1]
	    + mpn_add_n (rp + rn - sn - 1, rp + rn - sn - 1, rp + rn - 1, sn);
	}
      goto final_limbs;
    }
  else
    {
      while (rn >= 2 * mn - bn)
	{
	  rn -= sn;

	  for (i = 0; i < sn; i++)
	    rp[rn+i] = mpn_addmul_1 (rp + rn - mn + i, bp, bn, rp[rn+i]);
				     
	  hi = mpn_add_n (rp + rn - sn, rp + rn - sn, rp + rn, sn);
	  hi = cnd_add_n (hi, rp + rn - mn, bp, mn);
	  assert (hi == 0);
	}
    }

  if (rn > mn)
    {
    final_limbs:
      sn = rn - mn;
      
      for (i = 0; i < sn; i++)
	rp[mn+i] = mpn_addmul_1 (rp + i, bp, bn, rp[mn+i]);

      hi = mpn_add_n (rp + bn, rp + bn, rp + mn, sn);
      hi = sec_add_1 (rp + bn + sn, rp + bn + sn, mn - bn - sn, hi);
    }

  if (shift > 0)
    {
      /* Combine hi with top bits, add in */
      hi = (hi << shift) | (rp[mn-1] >> (GMP_NUMB_BITS - shift));
      rp[mn-1] = (rp[mn-1] & (((mp_limb_t) 1 << (GMP_NUMB_BITS - shift)) - 1))
	+ mpn_addmul_1 (rp, b_shifted, mn-1, hi);
    }
  else
    {
      hi = cnd_add_n (hi, rp, bp, mn);
      assert (hi == 0);
    }
}