tommath/bn_mp_prime_miller_rabin.c

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#include <tommath_private.h>
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#ifdef BN_MP_PRIME_MILLER_RABIN_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* 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, tstdenis82@gmail.com, http://libtom.org
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*/
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/* Miller-Rabin test of "a" to the base of "b" as described in
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* HAC pp. 139 Algorithm 4.24
*
* Sets result to 0 if definitely composite or 1 if probably prime.
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* Randomly the chance of error is no more than 1/4 and often
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* very much lower.
*/
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int mp_prime_miller_rabin(const mp_int *a, const mp_int *b, int *result)
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{
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mp_int n1, y, r;
int s, j, err;
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/* default */
*result = MP_NO;
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/* ensure b > 1 */
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if (mp_cmp_d(b, 1uL) != MP_GT) {
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return MP_VAL;
}
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/* get n1 = a - 1 */
if ((err = mp_init_copy(&n1, a)) != MP_OKAY) {
return err;
}
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if ((err = mp_sub_d(&n1, 1uL, &n1)) != MP_OKAY) {
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goto LBL_N1;
}
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/* set 2**s * r = n1 */
if ((err = mp_init_copy(&r, &n1)) != MP_OKAY) {
goto LBL_N1;
}
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/* count the number of least significant bits
* which are zero
*/
s = mp_cnt_lsb(&r);
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/* now divide n - 1 by 2**s */
if ((err = mp_div_2d(&r, s, &r, NULL)) != MP_OKAY) {
goto LBL_R;
}
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/* 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;
}
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/* if y != 1 and y != n1 do */
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if ((mp_cmp_d(&y, 1uL) != MP_EQ) && (mp_cmp(&y, &n1) != MP_EQ)) {
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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 */
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if (mp_cmp_d(&y, 1uL) == MP_EQ) {
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goto LBL_Y;
}
++j;
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}
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/* if y != n1 then composite */
if (mp_cmp(&y, &n1) != MP_EQ) {
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goto LBL_Y;
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}
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}
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/* probably prime now */
*result = MP_YES;
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LBL_Y:
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mp_clear(&y);
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LBL_R:
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mp_clear(&r);
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LBL_N1:
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mp_clear(&n1);
return err;
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}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */