94 lines
2.5 KiB
C
94 lines
2.5 KiB
C
#include "amplify_tommath_private.h"
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#ifdef AMPLIFY_BN_MP_GCD_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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/* Modifications Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. */
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/* Greatest Common Divisor using the binary method */
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amplify_mp_err amplify_mp_gcd(const amplify_mp_int *a, const amplify_mp_int *b, amplify_mp_int *c)
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{
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amplify_mp_int u, v;
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int k, u_lsb, v_lsb;
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amplify_mp_err err;
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/* either zero than gcd is the largest */
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if (AMPLIFY_MP_IS_ZERO(a)) {
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return amplify_mp_abs(b, c);
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}
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if (AMPLIFY_MP_IS_ZERO(b)) {
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return amplify_mp_abs(a, c);
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}
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/* get copies of a and b we can modify */
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if ((err = amplify_mp_init_copy(&u, a)) != AMPLIFY_MP_OKAY) {
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return err;
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}
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if ((err = amplify_mp_init_copy(&v, b)) != AMPLIFY_MP_OKAY) {
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goto LBL_U;
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}
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/* must be positive for the remainder of the algorithm */
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u.sign = v.sign = AMPLIFY_MP_ZPOS;
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/* B1. Find the common power of two for u and v */
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u_lsb = amplify_mp_cnt_lsb(&u);
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v_lsb = amplify_mp_cnt_lsb(&v);
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k = AMPLIFY_MP_MIN(u_lsb, v_lsb);
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if (k > 0) {
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/* divide the power of two out */
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if ((err = amplify_mp_div_2d(&u, k, &u, NULL)) != AMPLIFY_MP_OKAY) {
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goto LBL_V;
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}
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if ((err = amplify_mp_div_2d(&v, k, &v, NULL)) != AMPLIFY_MP_OKAY) {
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goto LBL_V;
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}
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}
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/* divide any remaining factors of two out */
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if (u_lsb != k) {
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if ((err = amplify_mp_div_2d(&u, u_lsb - k, &u, NULL)) != AMPLIFY_MP_OKAY) {
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goto LBL_V;
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}
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}
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if (v_lsb != k) {
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if ((err = amplify_mp_div_2d(&v, v_lsb - k, &v, NULL)) != AMPLIFY_MP_OKAY) {
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goto LBL_V;
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}
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}
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while (!AMPLIFY_MP_IS_ZERO(&v)) {
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/* make sure v is the largest */
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if (amplify_mp_cmp_mag(&u, &v) == AMPLIFY_MP_GT) {
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/* swap u and v to make sure v is >= u */
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amplify_mp_exch(&u, &v);
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}
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/* subtract smallest from largest */
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if ((err = amplify_s_mp_sub(&v, &u, &v)) != AMPLIFY_MP_OKAY) {
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goto LBL_V;
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}
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/* Divide out all factors of two */
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if ((err = amplify_mp_div_2d(&v, amplify_mp_cnt_lsb(&v), &v, NULL)) != AMPLIFY_MP_OKAY) {
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goto LBL_V;
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}
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}
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/* multiply by 2**k which we divided out at the beginning */
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if ((err = amplify_mp_mul_2d(&u, k, c)) != AMPLIFY_MP_OKAY) {
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goto LBL_V;
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}
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c->sign = AMPLIFY_MP_ZPOS;
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err = AMPLIFY_MP_OKAY;
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LBL_V:
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amplify_mp_clear(&u);
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LBL_U:
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amplify_mp_clear(&v);
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return err;
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}
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#endif
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