### view libtommath/bn_mp_prime_fermat.c @ 709:abd99ecd7ec2

use an empty writebuf rather than a NULL one
author Matt Johnston Wed, 20 Mar 2013 22:31:07 +0800 5ff8218bcee9 60fc6476e044
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```#include <tommath.h>
#ifdef BN_MP_PRIME_FERMAT_C
/* 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
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://math.libtomcrypt.com
*/

/* 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 = MP_NO;

/* 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 LBL_T;
}

/* is it equal to b? */
if (mp_cmp (&t, b) == MP_EQ) {
*result = MP_YES;
}

err = MP_OKAY;
LBL_T:mp_clear (&t);
return err;
}
#endif

/* \$Source: /cvs/libtom/libtommath/bn_mp_prime_fermat.c,v \$ */
/* \$Revision: 1.3 \$ */
/* \$Date: 2006/03/31 14:18:44 \$ */
```