### view libtommath/bn_mp_prime_miller_rabin.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_MP_PRIME_MILLER_RABIN_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* Miller-Rabin test of "a" to the base of "b" as described in
* HAC pp. 139 Algorithm 4.24
*
* Sets result to 0 if definitely composite or 1 if probably prime.
* Randomly the chance of error is no more than 1/4 and often
* very much lower.
*/
mp_err mp_prime_miller_rabin(const mp_int *a, const mp_int *b, mp_bool *result)
{
mp_int  n1, y, r;
mp_err  err;
int     s, j;

/* default */
*result = MP_NO;

/* ensure b > 1 */
if (mp_cmp_d(b, 1uL) != MP_GT) {
return MP_VAL;
}

/* get n1 = a - 1 */
if ((err = mp_init_copy(&n1, a)) != MP_OKAY) {
return err;
}
if ((err = mp_sub_d(&n1, 1uL, &n1)) != MP_OKAY) {
goto LBL_N1;
}

/* set 2**s * r = n1 */
if ((err = mp_init_copy(&r, &n1)) != MP_OKAY) {
goto LBL_N1;
}

/* count the number of least significant bits
* which are zero
*/
s = mp_cnt_lsb(&r);

/* now divide n - 1 by 2**s */
if ((err = mp_div_2d(&r, s, &r, NULL)) != MP_OKAY) {
goto LBL_R;
}

/* compute y = b**r mod a */
if ((err = mp_init(&y)) != MP_OKAY) {
goto LBL_R;
}
if ((err = mp_exptmod(b, &r, a, &y)) != MP_OKAY) {
goto LBL_Y;
}

/* if y != 1 and y != n1 do */
if ((mp_cmp_d(&y, 1uL) != MP_EQ) && (mp_cmp(&y, &n1) != MP_EQ)) {
j = 1;
/* while j <= s-1 and y != n1 */
while ((j <= (s - 1)) && (mp_cmp(&y, &n1) != MP_EQ)) {
if ((err = mp_sqrmod(&y, a, &y)) != MP_OKAY) {
goto LBL_Y;
}

/* if y == 1 then composite */
if (mp_cmp_d(&y, 1uL) == MP_EQ) {
goto LBL_Y;
}

++j;
}

/* if y != n1 then composite */
if (mp_cmp(&y, &n1) != MP_EQ) {
goto LBL_Y;
}
}

/* probably prime now */
*result = MP_YES;
LBL_Y:
mp_clear(&y);
LBL_R:
mp_clear(&r);
LBL_N1:
mp_clear(&n1);
return err;
}
#endif
```