53 lines
1.2 KiB
C
53 lines
1.2 KiB
C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is library that provides for multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library is 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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/* performs one Fermat test.
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*
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* If "a" were prime then b^a == b (mod a) since the order of
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* the multiplicative sub-group would be phi(a) = a-1. That means
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* it would be the same as b^(a mod (a-1)) == b^1 == b (mod a).
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*
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* Sets result to 1 if the congruence holds, or zero otherwise.
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*/
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int
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mp_prime_fermat (mp_int * a, mp_int * b, int *result)
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{
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mp_int t;
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int err;
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/* default to fail */
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*result = 0;
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/* init t */
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if ((err = mp_init (&t)) != MP_OKAY) {
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return err;
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}
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/* compute t = b^a mod a */
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if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) {
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goto __T;
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}
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/* is it equal to b? */
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if (mp_cmp (&t, b) == MP_EQ) {
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*result = 1;
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}
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err = MP_OKAY;
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__T:mp_clear (&t);
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return err;
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}
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