53 lines
1.0 KiB
C
53 lines
1.0 KiB
C
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#include "tommath_private.h"
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#ifdef MP_REDUCE_2K_L_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* reduces a modulo n where n is of the form 2**p - d
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This differs from reduce_2k since "d" can be larger
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than a single digit.
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*/
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mp_err mp_reduce_2k_l(mp_int *a, const mp_int *n, const mp_int *d)
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{
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mp_int q;
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mp_err err;
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int p;
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if ((err = mp_init(&q)) != MP_OKAY) {
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return err;
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}
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p = mp_count_bits(n);
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for (;;) {
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/* q = a/2**p, a = a mod 2**p */
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if ((err = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* q = q * d */
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if ((err = mp_mul(&q, d, &q)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* a = a + q */
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if ((err = s_mp_add(a, &q, a)) != MP_OKAY) {
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goto LBL_ERR;
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}
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if (mp_cmp_mag(a, n) == MP_LT) {
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break;
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}
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if ((err = s_mp_sub(a, n, a)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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LBL_ERR:
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mp_clear(&q);
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return err;
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}
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#endif
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