101 lines
2.4 KiB
C
101 lines
2.4 KiB
C
#include <tommath.h>
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#ifdef BN_MP_REDUCE_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@gmail.com, http://libtom.org
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*/
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/* reduces x mod m, assumes 0 < x < m**2, mu is
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* precomputed via mp_reduce_setup.
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* From HAC pp.604 Algorithm 14.42
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*/
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int mp_reduce (mp_int * x, mp_int * m, mp_int * mu)
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{
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mp_int q;
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int res, um = m->used;
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/* q = x */
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if ((res = mp_init_copy (&q, x)) != MP_OKAY) {
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return res;
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}
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/* q1 = x / b**(k-1) */
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mp_rshd (&q, um - 1);
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/* according to HAC this optimization is ok */
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if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) {
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if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) {
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goto CLEANUP;
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}
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} else {
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#ifdef BN_S_MP_MUL_HIGH_DIGS_C
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if ((res = s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) {
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goto CLEANUP;
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}
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#elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C)
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if ((res = fast_s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) {
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goto CLEANUP;
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}
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#else
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{
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res = MP_VAL;
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goto CLEANUP;
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}
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#endif
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}
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/* q3 = q2 / b**(k+1) */
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mp_rshd (&q, um + 1);
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/* x = x mod b**(k+1), quick (no division) */
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if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
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goto CLEANUP;
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}
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/* q = q * m mod b**(k+1), quick (no division) */
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if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) {
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goto CLEANUP;
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}
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/* x = x - q */
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if ((res = mp_sub (x, &q, x)) != MP_OKAY) {
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goto CLEANUP;
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}
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/* If x < 0, add b**(k+1) to it */
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if (mp_cmp_d (x, 0) == MP_LT) {
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mp_set (&q, 1);
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if ((res = mp_lshd (&q, um + 1)) != MP_OKAY)
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goto CLEANUP;
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if ((res = mp_add (x, &q, x)) != MP_OKAY)
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goto CLEANUP;
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}
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/* Back off if it's too big */
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while (mp_cmp (x, m) != MP_LT) {
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if ((res = s_mp_sub (x, m, x)) != MP_OKAY) {
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goto CLEANUP;
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}
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}
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CLEANUP:
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mp_clear (&q);
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return res;
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}
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#endif
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/* $Source: /cvs/libtom/libtommath/bn_mp_reduce.c,v $ */
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/* $Revision: 1.4 $ */
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/* $Date: 2006/12/28 01:25:13 $ */
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