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