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/* bignum.c
*
* bignum operations that are missing from gmp.
*/
/* nettle, low-level cryptographics library
*
* Copyright (C) 2001 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., 59 Temple Place - Suite 330, Boston,
* MA 02111-1307, USA.
*/
#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. */
unsigned
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 */
unsigned 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;
}
}
unsigned
nettle_mpz_sizeinbase_256_u(const mpz_t x)
{
return (mpz_sizeinbase(x,2) + 7) / 8;
}
static void
nettle_mpz_to_octets(unsigned length, uint8_t *s,
const mpz_t x, uint8_t sign)
{
uint8_t *dst = s + length - 1;
unsigned size = mpz_size(x);
unsigned i;
for (i = 0; i<size; i++)
{
mp_limb_t limb = mpz_getlimbn(x, i);
unsigned 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(unsigned 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 */
#ifdef 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))
#else
static void
nettle_mpz_from_octets(mpz_t x,
unsigned length, const uint8_t *s)
{
unsigned i;
mpz_set_ui(x, 0);
for (i = 0; i < length; i++)
{
mpz_mul_2exp(x, x, 8);
mpz_add_ui(x, x, s[i]);
}
}
#endif
void
nettle_mpz_set_str_256_u(mpz_t x,
unsigned length, const uint8_t *s)
{
nettle_mpz_from_octets(x, length, s);
}
void
nettle_mpz_init_set_str_256_u(mpz_t x,
unsigned 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,
unsigned 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,
unsigned length, const uint8_t *s)
{
mpz_init(x);
nettle_mpz_set_str_256_s(x, length, s);
}
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