120 lines
3.7 KiB
C
120 lines
3.7 KiB
C
#include "tommath_private.h"
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#ifdef BN_S_MP_INVMOD_SLOW_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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/* hac 14.61, pp608 */
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mp_err s_mp_invmod_slow(const mp_int *a, const mp_int *b, mp_int *c)
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{
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mp_int x, y, u, v, A, B, C, D;
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mp_err err;
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/* b cannot be negative */
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if ((b->sign == MP_NEG) || MP_IS_ZERO(b)) {
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return MP_VAL;
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}
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/* init temps */
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if ((err = mp_init_multi(&x, &y, &u, &v,
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&A, &B, &C, &D, NULL)) != MP_OKAY) {
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return err;
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}
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/* x = a, y = b */
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if ((err = mp_mod(a, b, &x)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_copy(b, &y)) != MP_OKAY) goto LBL_ERR;
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/* 2. [modified] if x,y are both even then return an error! */
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if (MP_IS_EVEN(&x) && MP_IS_EVEN(&y)) {
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err = MP_VAL;
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goto LBL_ERR;
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}
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/* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
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if ((err = mp_copy(&x, &u)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_copy(&y, &v)) != MP_OKAY) goto LBL_ERR;
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mp_set(&A, 1uL);
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mp_set(&D, 1uL);
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top:
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/* 4. while u is even do */
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while (MP_IS_EVEN(&u)) {
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/* 4.1 u = u/2 */
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if ((err = mp_div_2(&u, &u)) != MP_OKAY) goto LBL_ERR;
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/* 4.2 if A or B is odd then */
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if (MP_IS_ODD(&A) || MP_IS_ODD(&B)) {
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/* A = (A+y)/2, B = (B-x)/2 */
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if ((err = mp_add(&A, &y, &A)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_sub(&B, &x, &B)) != MP_OKAY) goto LBL_ERR;
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}
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/* A = A/2, B = B/2 */
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if ((err = mp_div_2(&A, &A)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_div_2(&B, &B)) != MP_OKAY) goto LBL_ERR;
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}
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/* 5. while v is even do */
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while (MP_IS_EVEN(&v)) {
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/* 5.1 v = v/2 */
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if ((err = mp_div_2(&v, &v)) != MP_OKAY) goto LBL_ERR;
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/* 5.2 if C or D is odd then */
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if (MP_IS_ODD(&C) || MP_IS_ODD(&D)) {
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/* C = (C+y)/2, D = (D-x)/2 */
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if ((err = mp_add(&C, &y, &C)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_sub(&D, &x, &D)) != MP_OKAY) goto LBL_ERR;
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}
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/* C = C/2, D = D/2 */
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if ((err = mp_div_2(&C, &C)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_div_2(&D, &D)) != MP_OKAY) goto LBL_ERR;
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}
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/* 6. if u >= v then */
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if (mp_cmp(&u, &v) != MP_LT) {
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/* u = u - v, A = A - C, B = B - D */
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if ((err = mp_sub(&u, &v, &u)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_sub(&A, &C, &A)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_sub(&B, &D, &B)) != MP_OKAY) goto LBL_ERR;
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} else {
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/* v - v - u, C = C - A, D = D - B */
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if ((err = mp_sub(&v, &u, &v)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_sub(&C, &A, &C)) != MP_OKAY) goto LBL_ERR;
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if ((err = mp_sub(&D, &B, &D)) != MP_OKAY) goto LBL_ERR;
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}
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/* if not zero goto step 4 */
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if (!MP_IS_ZERO(&u)) {
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goto top;
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}
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/* now a = C, b = D, gcd == g*v */
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/* if v != 1 then there is no inverse */
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if (mp_cmp_d(&v, 1uL) != MP_EQ) {
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err = MP_VAL;
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goto LBL_ERR;
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}
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/* if its too low */
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while (mp_cmp_d(&C, 0uL) == MP_LT) {
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if ((err = mp_add(&C, b, &C)) != MP_OKAY) goto LBL_ERR;
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}
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/* too big */
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while (mp_cmp_mag(&C, b) != MP_LT) {
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if ((err = mp_sub(&C, b, &C)) != MP_OKAY) goto LBL_ERR;
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}
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/* C is now the inverse */
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mp_exch(&C, c);
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err = MP_OKAY;
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LBL_ERR:
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mp_clear_multi(&x, &y, &u, &v, &A, &B, &C, &D, NULL);
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return err;
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}
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#endif
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