File: bignum.c

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/* bignum.c

   Bignum operations that are missing from gmp.

   Copyright (C) 2001 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 <string.h>

#include "bignum.h"

/* Two's complement negation means that -x = ~x + 1, ~x = -(x+1),
 * and we use that x = ~~x = ~(-x-1).
 *
 * Examples:
 *
 *   x  ~x = -x+1     ~~x = x
 *  -1          0          ff
 *  -2          1          fe
 * -7f         7e          81
 * -80         7f          80
 * -81         80        ff7f
 */

/* Including extra sign bit, if needed. Also one byte for zero. */
size_t
nettle_mpz_sizeinbase_256_s(const mpz_t x)
{
  if (mpz_sgn(x) >= 0)
    return 1 + mpz_sizeinbase(x, 2) / 8;
  else
    {
      /* We'll output ~~x, so we need as many bits as for ~x */
      size_t size;
      mpz_t c;

      mpz_init(c);
      mpz_com(c, x); /* Same as c = - x - 1 = |x| + 1 */
      size = 1 + mpz_sizeinbase(c,2) / 8;
      mpz_clear(c);

      return size;
    }
}

size_t
nettle_mpz_sizeinbase_256_u(const mpz_t x)
{
  return (mpz_sizeinbase(x,2) + 7) / 8;
}

static void
nettle_mpz_to_octets(size_t length, uint8_t *s,
		     const mpz_t x, uint8_t sign)
{
  uint8_t *dst = s + length - 1;
  size_t size = mpz_size(x);
  size_t i;
  
  for (i = 0; i<size; i++)
    {
      mp_limb_t limb = mpz_getlimbn(x, i);
      size_t j;

      for (j = 0; length && j < sizeof(mp_limb_t); j++)
        {
          *dst-- = sign ^ (limb & 0xff);
          limb >>= 8;
          length--;
	}
    }
  
  if (length)
    memset(s, sign, length);
}

void
nettle_mpz_get_str_256(size_t length, uint8_t *s, const mpz_t x)
{
  if (!length)
    {
      /* x must be zero */
      assert(!mpz_sgn(x));
      return;
    }

  if (mpz_sgn(x) >= 0)
    {
      assert(nettle_mpz_sizeinbase_256_u(x) <= length);
      nettle_mpz_to_octets(length, s, x, 0);
    }
  else
    {
      mpz_t c;
      mpz_init(c);
      mpz_com(c, x);

      assert(nettle_mpz_sizeinbase_256_u(c) <= length);
      nettle_mpz_to_octets(length, s, c, 0xff);

      mpz_clear(c);
    }
}

/* Converting from strings */

/* mpz_import was introduced in GMP-4.1 */
#define nettle_mpz_from_octets(x, length, s) \
   mpz_import((x), (length), 1, 1, 0, 0, (s))

void
nettle_mpz_set_str_256_u(mpz_t x,
			 size_t length, const uint8_t *s)
{
  nettle_mpz_from_octets(x, length, s);
}

void
nettle_mpz_init_set_str_256_u(mpz_t x,
			      size_t length, const uint8_t *s)
{
  mpz_init(x);
  nettle_mpz_from_octets(x, length, s);
}

void
nettle_mpz_set_str_256_s(mpz_t x,
			 size_t length, const uint8_t *s)
{
  if (!length)
    {
      mpz_set_ui(x, 0);
      return;
    }
  
  nettle_mpz_from_octets(x, length, s);

  if (s[0] & 0x80)
    {
      mpz_t t;

      mpz_init_set_ui(t, 1);
      mpz_mul_2exp(t, t, length*8);
      mpz_sub(x, x, t);
      mpz_clear(t);
    }
}

void
nettle_mpz_init_set_str_256_s(mpz_t x,
			      size_t length, const uint8_t *s)
{
  mpz_init(x);
  nettle_mpz_set_str_256_s(x, length, s);
}