99 lines
2.2 KiB
C
99 lines
2.2 KiB
C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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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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*
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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
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
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#include <tommath.h>
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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
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*
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* 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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*/
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int
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mp_prime_miller_rabin (mp_int * a, mp_int * b, int *result)
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{
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mp_int n1, y, r;
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int s, j, err;
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/* default */
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*result = 0;
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/* ensure b > 1 */
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if (mp_cmp_d(b, 1) != MP_GT) {
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return MP_VAL;
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}
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/* get n1 = a - 1 */
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if ((err = mp_init_copy (&n1, a)) != MP_OKAY) {
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return err;
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}
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if ((err = mp_sub_d (&n1, 1, &n1)) != MP_OKAY) {
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goto __N1;
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}
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/* set 2**s * r = n1 */
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if ((err = mp_init_copy (&r, &n1)) != MP_OKAY) {
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goto __N1;
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}
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/* count the number of least significant bits
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* which are zero
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*/
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s = mp_cnt_lsb(&r);
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/* now divide n - 1 by 2**s */
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if ((err = mp_div_2d (&r, s, &r, NULL)) != MP_OKAY) {
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goto __R;
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}
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/* compute y = b**r mod a */
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if ((err = mp_init (&y)) != MP_OKAY) {
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goto __R;
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}
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if ((err = mp_exptmod (b, &r, a, &y)) != MP_OKAY) {
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goto __Y;
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}
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/* if y != 1 and y != n1 do */
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if (mp_cmp_d (&y, 1) != MP_EQ && mp_cmp (&y, &n1) != MP_EQ) {
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j = 1;
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/* while j <= s-1 and y != n1 */
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while ((j <= (s - 1)) && mp_cmp (&y, &n1) != MP_EQ) {
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if ((err = mp_sqrmod (&y, a, &y)) != MP_OKAY) {
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goto __Y;
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}
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/* if y == 1 then composite */
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if (mp_cmp_d (&y, 1) == MP_EQ) {
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goto __Y;
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}
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++j;
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}
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/* if y != n1 then composite */
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if (mp_cmp (&y, &n1) != MP_EQ) {
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goto __Y;
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}
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}
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/* probably prime now */
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*result = 1;
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__Y:mp_clear (&y);
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__R:mp_clear (&r);
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__N1:mp_clear (&n1);
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return err;
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}
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