File: ecc-mul-g.c

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/* ecc-mul-g.c

   Copyright (C) 2013 Niels Möller

   This file is part of GNU Nettle.

   GNU Nettle is free software: you can redistribute it and/or
   modify it under the terms of either:

     * the GNU Lesser General Public License as published by the Free
       Software Foundation; either version 3 of the License, or (at your
       option) any later version.

   or

     * the GNU General Public License as published by the Free
       Software Foundation; either version 2 of the License, or (at your
       option) any later version.

   or both in parallel, as here.

   GNU Nettle 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
   General Public License for more details.

   You should have received copies of the GNU General Public License and
   the GNU Lesser General Public License along with this program.  If
   not, see http://www.gnu.org/licenses/.
*/

/* 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"

void
ecc_mul_g (const struct ecc_curve *ecc, mp_limb_t *r,
	   const mp_limb_t *np, mp_limb_t *scratch)
{
  /* Scratch need determined by the ecc_add_jja call. Current total is
     9 * ecc->p.size, at most 648 bytes. */
#define tp scratch
#define scratch_out (scratch + 3*ecc->p.size)

  unsigned k, c;
  unsigned i, j;
  unsigned bit_rows;

  int is_zero;

  k = ecc->pippenger_k;
  c = ecc->pippenger_c;

  bit_rows = (ecc->p.bit_size + k - 1) / k;
  
  mpn_zero (r, 3*ecc->p.size);
  
  for (i = k, is_zero = 1; i-- > 0; )
    {
      ecc_dup_jj (ecc, r, r, scratch);
      for (j = 0; j * c < bit_rows; j++)
	{
	  unsigned bits;
	  /* Avoid the mp_bitcnt_t type for compatibility with older GMP
	     versions. */
	  unsigned bit_index;
	  
	  /* Extract c bits from n, stride k, starting at i + kcj,
	     ending at i + k (cj + c - 1)*/
	  for (bits = 0, bit_index = i + k*(c*j+c); bit_index > i + k*c*j; )
	    {
	      mp_size_t limb_index;
	      unsigned shift;
	      
	      bit_index -= k;

	      limb_index = bit_index / GMP_NUMB_BITS;
	      if (limb_index >= ecc->p.size)
		continue;

	      shift = bit_index % GMP_NUMB_BITS;
	      bits = (bits << 1) | ((np[limb_index] >> shift) & 1);
	    }
	  sec_tabselect (tp, 2*ecc->p.size,
			 (ecc->pippenger_table
			  + (2*ecc->p.size * (mp_size_t) j << c)),
			 1<<c, bits);
	  cnd_copy (is_zero, r, tp, 2*ecc->p.size);
	  cnd_copy (is_zero, r + 2*ecc->p.size, ecc->unit, ecc->p.size);
	  
	  ecc_add_jja (ecc, tp, r, tp, scratch_out);
	  /* Use the sum when valid. ecc_add_jja produced garbage if
	     is_zero != 0 or bits == 0, . */	  
	  cnd_copy (bits & (is_zero - 1), r, tp, 3*ecc->p.size);
	  is_zero &= (bits == 0);
	}
    }
#undef tp
#undef scratch_out
}