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annotate libtommath/bn_mp_prime_fermat.c @ 1671:5c8913b7464c

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Mention Ed25519 in dropbearkey man (#94)
author Vladislav Grishenko <themiron@users.noreply.github.com>
date Mon, 25 May 2020 18:28:27 +0500
parents f52919ffd3b1
children 1051e4eea25a
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1 #include "tommath_private.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 * SPDX-License-Identifier: Unlicense
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13 */
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14
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15 /* performs one Fermat test.
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16 *
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17 * If "a" were prime then b**a == b (mod a) since the order of
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18 * the multiplicative sub-group would be phi(a) = a-1. That means
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19 * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
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20 *
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21 * Sets result to 1 if the congruence holds, or zero otherwise.
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22 */
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23 int mp_prime_fermat(const mp_int *a, const mp_int *b, int *result)
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24 {
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25 mp_int t;
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26 int err;
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27
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28 /* default to composite */
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29 *result = MP_NO;
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30
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31 /* ensure b > 1 */
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32 if (mp_cmp_d(b, 1uL) != MP_GT) {
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33 return MP_VAL;
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34 }
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35
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36 /* init t */
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37 if ((err = mp_init(&t)) != MP_OKAY) {
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38 return err;
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39 }
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40
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41 /* compute t = b**a mod a */
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42 if ((err = mp_exptmod(b, a, a, &t)) != MP_OKAY) {
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43 goto LBL_T;
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44 }
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46 /* is it equal to b? */
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47 if (mp_cmp(&t, b) == MP_EQ) {
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48 *result = MP_YES;
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49 }
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50
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51 err = MP_OKAY;
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52 LBL_T:
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53 mp_clear(&t);
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54 return err;
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55 }
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56 #endif
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57
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58 /* ref: HEAD -> master, tag: v1.1.0 */
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59 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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60 /* commit time: 2019-01-28 20:32:32 +0100 */