1 #include "tommath_private.h"
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
4 /* SPDX-License-Identifier: Unlicense */
6 /* reduces x mod m, assumes 0 < x < m**2, mu is
7 * precomputed via mp_reduce_setup.
8 * From HAC pp.604 Algorithm 14.42
10 mp_err mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
17 if ((err = mp_init_copy(&q, x)) != MP_OKAY) {
21 /* q1 = x / b**(k-1) */
24 /* according to HAC this optimization is ok */
25 if ((mp_digit)um > ((mp_digit)1 << (MP_DIGIT_BIT - 1))) {
26 if ((err = mp_mul(&q, mu, &q)) != MP_OKAY) {
29 } else if (MP_HAS(S_MP_MUL_HIGH_DIGS)) {
30 if ((err = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
33 } else if (MP_HAS(S_MP_MUL_HIGH_DIGS_FAST)) {
34 if ((err = s_mp_mul_high_digs_fast(&q, mu, &q, um)) != MP_OKAY) {
42 /* q3 = q2 / b**(k+1) */
45 /* x = x mod b**(k+1), quick (no division) */
46 if ((err = mp_mod_2d(x, MP_DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
50 /* q = q * m mod b**(k+1), quick (no division) */
51 if ((err = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
56 if ((err = mp_sub(x, &q, x)) != MP_OKAY) {
60 /* If x < 0, add b**(k+1) to it */
61 if (mp_cmp_d(x, 0uL) == MP_LT) {
63 if ((err = mp_lshd(&q, um + 1)) != MP_OKAY) {
66 if ((err = mp_add(x, &q, x)) != MP_OKAY) {
71 /* Back off if it's too big */
72 while (mp_cmp(x, m) != MP_LT) {
73 if ((err = s_mp_sub(x, m, x)) != MP_OKAY) {