ecc-mul-a-eh.c

/* ecc-mul-a-eh.c

   Copyright (C) 2013, 2014 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/.
*/

#if HAVE_CONFIG_H
# include "config.h"
#endif

#include <assert.h>

#include "ecc.h"
#include "ecc-internal.h"

/* Binary algorithm needs 6*ecc->p.size + scratch for ecc_add_ehh,
   total 10 ecc->p.size

   Window algorithm needs (3<<w) * ecc->p.size for the table,
   3*ecc->p.size for a temporary point, and scratch for
   ecc_add_ehh. */

#if ECC_MUL_A_EH_WBITS == 0
void
ecc_mul_a_eh (const struct ecc_curve *ecc,
	      mp_limb_t *r,
	      const mp_limb_t *np, const mp_limb_t *p,
	      mp_limb_t *scratch)
{
#define pe scratch
#define tp (scratch + 3*ecc->p.size)
#define scratch_out (scratch + 6*ecc->p.size)

  unsigned i;

  ecc_a_to_j (ecc, pe, p);

  /* x = 0, y = 1, z = 1 */
  mpn_zero (r, 3*ecc->p.size);
  r[ecc->p.size] = r[2*ecc->p.size] = 1;
  
  for (i = ecc->p.size; i-- > 0; )
    {
      mp_limb_t w = np[i] << (GMP_LIMB_BITS - GMP_NUMB_BITS);
      unsigned j;

      for (j = 0; j < GMP_NUMB_BITS; j++, w <<= 1)
	{
	  int bit;
	  ecc->dup (ecc, r, r, scratch_out);
	  ecc->add_hh (ecc, tp, r, pe, scratch_out);

	  bit = w >> (GMP_LIMB_BITS - 1);
	  /* If we had a one-bit, use the sum. */
	  cnd_copy (bit, r, tp, 3*ecc->p.size);
	}
    }
}
#else /* ECC_MUL_A_EH_WBITS > 1 */

#define TABLE_SIZE (1U << ECC_MUL_A_EH_WBITS)
#define TABLE_MASK (TABLE_SIZE - 1)

#define TABLE(j) (table + (j) * 3*ecc->p.size)

static void
table_init (const struct ecc_curve *ecc,
	    mp_limb_t *table, unsigned bits,
	    const mp_limb_t *p,
	    mp_limb_t *scratch)
{
  unsigned size = 1 << bits;
  unsigned j;

  mpn_zero (TABLE(0), 3*ecc->p.size);
  TABLE(0)[ecc->p.size] = TABLE(0)[2*ecc->p.size] = 1;

  ecc_a_to_j (ecc, TABLE(1), p);

  for (j = 2; j < size; j += 2)
    {
      ecc->dup (ecc, TABLE(j), TABLE(j/2), scratch);
      ecc->add_hh (ecc, TABLE(j+1), TABLE(j), TABLE(1), scratch);
    }
}

void
ecc_mul_a_eh (const struct ecc_curve *ecc,
	      mp_limb_t *r,
	      const mp_limb_t *np, const mp_limb_t *p,
	      mp_limb_t *scratch)
{
#define tp scratch
#define table (scratch + 3*ecc->p.size)
  mp_limb_t *scratch_out = table + (3*ecc->p.size << ECC_MUL_A_EH_WBITS);

  mp_bitcnt_t blocks = (ecc->p.bit_size + ECC_MUL_A_EH_WBITS - 1) / ECC_MUL_A_EH_WBITS;
  mp_bitcnt_t bit_index = (blocks-1) * ECC_MUL_A_EH_WBITS;

  mp_size_t limb_index = bit_index / GMP_NUMB_BITS;
  unsigned shift = bit_index % GMP_NUMB_BITS;
  mp_limb_t w, bits;

  table_init (ecc, table, ECC_MUL_A_EH_WBITS, p, scratch_out);

  w = np[limb_index];
  bits = w >> shift;
  if (limb_index < ecc->p.size - 1)
    bits |= np[limb_index + 1] << (GMP_NUMB_BITS - shift);

  assert (bits < TABLE_SIZE);

  mpn_sec_tabselect (r, table, 3*ecc->p.size, TABLE_SIZE, bits);

  for (;;)
    {
      unsigned j;
      if (shift >= ECC_MUL_A_EH_WBITS)
	{
	  shift -= ECC_MUL_A_EH_WBITS;
	  bits = w >> shift;
	}
      else
	{
	  if (limb_index == 0)
	    {
	      assert (shift == 0);
	      break;
	    }
	  bits = w << (ECC_MUL_A_EH_WBITS - shift);
	  w = np[--limb_index];
	  shift = shift + GMP_NUMB_BITS - ECC_MUL_A_EH_WBITS;
	  bits |= w >> shift;
	}
      for (j = 0; j < ECC_MUL_A_EH_WBITS; j++)
	ecc->dup (ecc, r, r, scratch_out);

      bits &= TABLE_MASK;
      mpn_sec_tabselect (tp, table, 3*ecc->p.size, TABLE_SIZE, bits);
      ecc->add_hhh (ecc, r, r, tp, scratch_out);
    }
#undef table
#undef tp
}

#endif /* ECC_MUL_A_EH_WBITS > 1 */