### view libtommath/bn_s_mp_invmod_fast.c @ 1734:73646de50f13DROPBEAR_2020.80

version 2020.80
author Matt Johnston Fri, 26 Jun 2020 21:45:59 +0800 1051e4eea25a
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line source
```#include "tommath_private.h"
#ifdef BN_S_MP_INVMOD_FAST_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* computes the modular inverse via binary extended euclidean algorithm,
* that is c = 1/a mod b
*
* Based on slow invmod except this is optimized for the case where b is
* odd as per HAC Note 14.64 on pp. 610
*/
mp_err s_mp_invmod_fast(const mp_int *a, const mp_int *b, mp_int *c)
{
mp_int  x, y, u, v, B, D;
mp_sign neg;
mp_err  err;

/* 2. [modified] b must be odd   */
if (MP_IS_EVEN(b)) {
return MP_VAL;
}

/* init all our temps */
if ((err = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) {
return err;
}

/* x == modulus, y == value to invert */
if ((err = mp_copy(b, &x)) != MP_OKAY)                         goto LBL_ERR;

/* we need y = |a| */
if ((err = mp_mod(a, b, &y)) != MP_OKAY)                       goto LBL_ERR;

/* if one of x,y is zero return an error! */
if (MP_IS_ZERO(&x) || MP_IS_ZERO(&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(&D, 1uL);

top:
/* 4.  while u is even do */
while (MP_IS_EVEN(&u)) {
/* 4.1 u = u/2 */
if ((err = mp_div_2(&u, &u)) != MP_OKAY)                    goto LBL_ERR;

/* 4.2 if B is odd then */
if (MP_IS_ODD(&B)) {
if ((err = mp_sub(&B, &x, &B)) != MP_OKAY)               goto LBL_ERR;
}
/* B = B/2 */
if ((err = mp_div_2(&B, &B)) != MP_OKAY)                    goto LBL_ERR;
}

/* 5.  while v is even do */
while (MP_IS_EVEN(&v)) {
/* 5.1 v = v/2 */
if ((err = mp_div_2(&v, &v)) != MP_OKAY)                    goto LBL_ERR;

/* 5.2 if D is odd then */
if (MP_IS_ODD(&D)) {
/* D = (D-x)/2 */
if ((err = mp_sub(&D, &x, &D)) != MP_OKAY)               goto LBL_ERR;
}
/* D = D/2 */
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, B = B - D */
if ((err = mp_sub(&u, &v, &u)) != MP_OKAY)                  goto LBL_ERR;

if ((err = mp_sub(&B, &D, &B)) != MP_OKAY)                  goto LBL_ERR;
} else {
/* v - v - u, D = D - B */
if ((err = mp_sub(&v, &u, &v)) != MP_OKAY)                  goto LBL_ERR;

if ((err = mp_sub(&D, &B, &D)) != MP_OKAY)                  goto LBL_ERR;
}

/* if not zero goto step 4 */
if (!MP_IS_ZERO(&u)) {
goto top;
}

/* 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;
}

/* b is now the inverse */
neg = a->sign;
while (D.sign == MP_NEG) {
if ((err = mp_add(&D, b, &D)) != MP_OKAY)                   goto LBL_ERR;
}

/* too big */
while (mp_cmp_mag(&D, b) != MP_LT) {
if ((err = mp_sub(&D, b, &D)) != MP_OKAY)                   goto LBL_ERR;
}

mp_exch(&D, c);
c->sign = neg;
err = MP_OKAY;

LBL_ERR:
mp_clear_multi(&x, &y, &u, &v, &B, &D, NULL);
return err;
}
#endif
```