Mercurial > dropbear

annotate libtommath/bn_mp_prime_fermat.c @ 295:03f65e461915 DROPBEAR_0.48.1

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0.48.1
author Matt Johnston <matt@ucc.asn.au>
date Sat, 11 Mar 2006 14:59:34 +0000
parents eed26cff980b
children 5ff8218bcee9
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284
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1 #include <tommath.h>
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2 #ifdef BN_MP_PRIME_FERMAT_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.org
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16 */
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17
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18 /* performs one Fermat test.
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19 *
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20 * If "a" were prime then b**a == b (mod a) since the order of
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21 * the multiplicative sub-group would be phi(a) = a-1. That means
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22 * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
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23 *
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24 * Sets result to 1 if the congruence holds, or zero otherwise.
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25 */
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26 int mp_prime_fermat (mp_int * a, mp_int * b, int *result)
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27 {
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28 mp_int t;
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29 int err;
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30
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31 /* default to composite */
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32 *result = MP_NO;
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33
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34 /* ensure b > 1 */
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35 if (mp_cmp_d(b, 1) != MP_GT) {
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36 return MP_VAL;
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37 }
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38
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39 /* init t */
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40 if ((err = mp_init (&t)) != MP_OKAY) {
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41 return err;
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42 }
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43
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44 /* compute t = b**a mod a */
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45 if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) {
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46 goto LBL_T;
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47 }
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48
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49 /* is it equal to b? */
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50 if (mp_cmp (&t, b) == MP_EQ) {
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51 *result = MP_YES;
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52 }
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53
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54 err = MP_OKAY;
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55 LBL_T:mp_clear (&t);
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56 return err;
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57 }
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58 #endif