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