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#include "tommath_private.h"
#ifdef BN_MP_REDUCE_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is a library that provides multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library was designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* SPDX-License-Identifier: Unlicense
*/
/* reduces x mod m, assumes 0 < x < m**2, mu is
* precomputed via mp_reduce_setup.
* From HAC pp.604 Algorithm 14.42
*/
int mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
{
mp_int q;
int res, um = m->used;
/* q = x */
if ((res = mp_init_copy(&q, x)) != MP_OKAY) {
return res;
}
/* q1 = x / b**(k-1) */
mp_rshd(&q, um - 1);
/* according to HAC this optimization is ok */
if ((mp_digit)um > ((mp_digit)1 << (DIGIT_BIT - 1))) {
if ((res = mp_mul(&q, mu, &q)) != MP_OKAY) {
goto CLEANUP;
}
} else {
#ifdef BN_S_MP_MUL_HIGH_DIGS_C
if ((res = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
goto CLEANUP;
}
#elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C)
if ((res = fast_s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
goto CLEANUP;
}
#else
{
res = MP_VAL;
goto CLEANUP;
}
#endif
}
/* q3 = q2 / b**(k+1) */
mp_rshd(&q, um + 1);
/* x = x mod b**(k+1), quick (no division) */
if ((res = mp_mod_2d(x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
goto CLEANUP;
}
/* q = q * m mod b**(k+1), quick (no division) */
if ((res = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
goto CLEANUP;
}
/* x = x - q */
if ((res = mp_sub(x, &q, x)) != MP_OKAY) {
goto CLEANUP;
}
/* If x < 0, add b**(k+1) to it */
if (mp_cmp_d(x, 0uL) == MP_LT) {
mp_set(&q, 1uL);
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) {
if ((res = s_mp_sub(x, m, x)) != MP_OKAY) {
goto CLEANUP;
}
}
CLEANUP:
mp_clear(&q);
return res;
}
#endif
/* ref: HEAD -> master, tag: v1.1.0 */
/* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
/* commit time: 2019-01-28 20:32:32 +0100 */
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