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

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

/* hac 14.61, pp608 */
mp_err s_mp_invmod_slow(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_IS_ZERO(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_IS_EVEN(&x) && MP_IS_EVEN(&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);

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 A or B is odd then */
if (MP_IS_ODD(&A) || MP_IS_ODD(&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_IS_EVEN(&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_IS_ODD(&C) || MP_IS_ODD(&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 */
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;
}

/* if its too low */
while (mp_cmp_d(&C, 0uL) == MP_LT) {
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);
err = MP_OKAY;
LBL_ERR:
mp_clear_multi(&x, &y, &u, &v, &A, &B, &C, &D, NULL);
return err;
}
#endif
```