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annotate tomsfastmath/src/numtheory/fp_prime_miller_rabin.c @ 643:a362b62d38b2 dropbear-tfm

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Add tomsfastmath from git rev bfa4582842bc3bab42e4be4aed5703437049502a with Makefile.in renamed
author Matt Johnston <matt@ucc.asn.au>
date Wed, 23 Nov 2011 18:10:20 +0700
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a362b62d38b2 Add tomsfastmath from git rev bfa4582842bc3bab42e4be4aed5703437049502a
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1 /* TomsFastMath, a fast ISO C bignum library.
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2 *
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3 * This project is meant to fill in where LibTomMath
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4 * falls short. That is speed ;-)
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5 *
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6 * This project is public domain and free for all purposes.
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7 *
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8 * Tom St Denis, [email protected]
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9 */
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10 #include <tfm.h>
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12 /* Miller-Rabin test of "a" to the base of "b" as described in
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13 * HAC pp. 139 Algorithm 4.24
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14 *
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15 * Sets result to 0 if definitely composite or 1 if probably prime.
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16 * Randomly the chance of error is no more than 1/4 and often
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17 * very much lower.
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18 */
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19 void fp_prime_miller_rabin (fp_int * a, fp_int * b, int *result)
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20 {
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21 fp_int n1, y, r;
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22 int s, j;
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24 /* default */
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25 *result = FP_NO;
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27 /* ensure b > 1 */
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28 if (fp_cmp_d(b, 1) != FP_GT) {
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29 return;
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30 }
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32 /* get n1 = a - 1 */
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33 fp_init_copy(&n1, a);
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34 fp_sub_d(&n1, 1, &n1);
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36 /* set 2**s * r = n1 */
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37 fp_init_copy(&r, &n1);
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39 /* count the number of least significant bits
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40 * which are zero
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41 */
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42 s = fp_cnt_lsb(&r);
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44 /* now divide n - 1 by 2**s */
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45 fp_div_2d (&r, s, &r, NULL);
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47 /* compute y = b**r mod a */
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48 fp_init(&y);
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49 fp_exptmod(b, &r, a, &y);
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51 /* if y != 1 and y != n1 do */
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52 if (fp_cmp_d (&y, 1) != FP_EQ && fp_cmp (&y, &n1) != FP_EQ) {
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53 j = 1;
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54 /* while j <= s-1 and y != n1 */
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55 while ((j <= (s - 1)) && fp_cmp (&y, &n1) != FP_EQ) {
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56 fp_sqrmod (&y, a, &y);
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57
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58 /* if y == 1 then composite */
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59 if (fp_cmp_d (&y, 1) == FP_EQ) {
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60 return;
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61 }
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62 ++j;
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63 }
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65 /* if y != n1 then composite */
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66 if (fp_cmp (&y, &n1) != FP_EQ) {
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67 return;
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68 }
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69 }
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70
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71 /* probably prime now */
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72 *result = FP_YES;
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73 }
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74
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75 /* $Source$ */
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76 /* $Revision$ */
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77 /* $Date$ */