/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is a library that provides multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library was designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
*/
#include <tommath.h>
/* performs one Fermat test.
*
* If "a" were prime then b**a == b (mod a) since the order of
* the multiplicative sub-group would be phi(a) = a-1. That means
* it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
*
* Sets result to 1 if the congruence holds, or zero otherwise.
*/
int
mp_prime_fermat (mp_int * a, mp_int * b, int *result)
{
mp_int t;
int err;
/* default to composite */
*result = 0;
/* ensure b > 1 */
if (mp_cmp_d(b, 1) != MP_GT) {
return MP_VAL;
}
/* init t */
if ((err = mp_init (&t)) != MP_OKAY) {
return err;
}
/* compute t = b**a mod a */
if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) {
goto __T;
}
/* is it equal to b? */
if (mp_cmp (&t, b) == MP_EQ) {
*result = 1;
}
err = MP_OKAY;
__T:mp_clear (&t);
return err;
}
