tommath/bn_mp_reduce.c

101 lines
2.3 KiB
C
Raw Normal View History

2004-10-29 18:07:18 -04:00
#include <tommath.h>
#ifdef BN_MP_REDUCE_C
2003-02-28 11:08:34 -05:00
/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
2003-08-04 21:24:44 -04:00
* LibTomMath is a library that provides multiple-precision
2003-02-28 11:08:34 -05:00
* integer arithmetic as well as number theoretic functionality.
*
2003-08-04 21:24:44 -04:00
* The library was designed directly after the MPI library by
2003-02-28 11:08:34 -05:00
* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
2007-04-18 05:58:18 -04:00
* Tom St Denis, tomstdenis@gmail.com, http://libtom.org
2003-02-28 11:08:34 -05:00
*/
2003-05-29 09:35:26 -04:00
/* reduces x mod m, assumes 0 < x < m**2, mu is
* precomputed via mp_reduce_setup.
2003-05-17 08:33:54 -04:00
* From HAC pp.604 Algorithm 14.42
2003-02-28 11:08:34 -05:00
*/
2005-02-12 03:40:15 -05:00
int mp_reduce (mp_int * x, mp_int * m, mp_int * mu)
2003-02-28 11:08:34 -05:00
{
2003-02-28 11:09:08 -05:00
mp_int q;
int res, um = m->used;
2003-02-28 11:08:34 -05:00
2003-05-29 09:35:26 -04:00
/* q = x */
2003-02-28 11:08:34 -05:00
if ((res = mp_init_copy (&q, x)) != MP_OKAY) {
return res;
}
2003-05-29 09:35:26 -04:00
/* q1 = x / b**(k-1) */
2003-05-17 08:33:54 -04:00
mp_rshd (&q, um - 1);
2003-02-28 11:08:34 -05:00
2003-06-19 06:04:50 -04:00
/* according to HAC this optimization is ok */
if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) {
2003-02-28 11:08:34 -05:00
if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) {
goto CLEANUP;
}
} else {
2004-10-29 18:07:18 -04:00
#ifdef BN_S_MP_MUL_HIGH_DIGS_C
2005-03-12 06:55:11 -05:00
if ((res = s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) {
2003-02-28 11:08:34 -05:00
goto CLEANUP;
}
2004-10-29 18:07:18 -04:00
#elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C)
2005-03-12 06:55:11 -05:00
if ((res = fast_s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) {
2004-10-29 18:07:18 -04:00
goto CLEANUP;
}
#else
{
res = MP_VAL;
goto CLEANUP;
}
#endif
2003-02-28 11:08:34 -05:00
}
2003-05-29 09:35:26 -04:00
/* q3 = q2 / b**(k+1) */
2003-05-17 08:33:54 -04:00
mp_rshd (&q, um + 1);
2003-02-28 11:08:34 -05:00
2003-05-29 09:35:26 -04:00
/* x = x mod b**(k+1), quick (no division) */
2003-02-28 11:08:34 -05:00
if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
goto CLEANUP;
}
2003-05-29 09:35:26 -04:00
/* q = q * m mod b**(k+1), quick (no division) */
2003-02-28 11:08:34 -05:00
if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) {
goto CLEANUP;
}
/* x = x - q */
2003-05-17 08:33:54 -04:00
if ((res = mp_sub (x, &q, x)) != MP_OKAY) {
2003-02-28 11:08:34 -05:00
goto CLEANUP;
2003-05-17 08:33:54 -04:00
}
2003-02-28 11:08:34 -05:00
2003-05-29 09:35:26 -04:00
/* If x < 0, add b**(k+1) to it */
2003-02-28 11:08:34 -05:00
if (mp_cmp_d (x, 0) == MP_LT) {
mp_set (&q, 1);
if ((res = mp_lshd (&q, um + 1)) != MP_OKAY)
goto CLEANUP;
if ((res = mp_add (x, &q, x)) != MP_OKAY)
goto CLEANUP;
}
/* Back off if it's too big */
while (mp_cmp (x, m) != MP_LT) {
2003-05-17 08:33:54 -04:00
if ((res = s_mp_sub (x, m, x)) != MP_OKAY) {
2003-06-19 06:04:50 -04:00
goto CLEANUP;
2003-05-17 08:33:54 -04:00
}
2003-02-28 11:08:34 -05:00
}
2003-05-29 09:35:26 -04:00
2003-02-28 11:08:34 -05:00
CLEANUP:
mp_clear (&q);
return res;
}
2004-10-29 18:07:18 -04:00
#endif
2005-08-01 12:37:28 -04:00
/* $Source$ */
/* $Revision$ */
/* $Date$ */