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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 /* calc a value mod 2**b */ |
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18 int |
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19 mp_mod_2d (mp_int * a, int b, mp_int * c) |
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20 { |
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21 int x, res; |
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22 |
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23 /* if b is <= 0 then zero the int */ |
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24 if (b <= 0) { |
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25 mp_zero (c); |
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26 return MP_OKAY; |
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27 } |
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28 |
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29 /* if the modulus is larger than the value than return */ |
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30 if (b > (int) (a->used * DIGIT_BIT)) { |
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31 res = mp_copy (a, c); |
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32 return res; |
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33 } |
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34 |
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35 /* copy */ |
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36 if ((res = mp_copy (a, c)) != MP_OKAY) { |
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37 return res; |
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38 } |
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39 |
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40 /* zero digits above the last digit of the modulus */ |
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41 for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) { |
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42 c->dp[x] = 0; |
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43 } |
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44 /* clear the digit that is not completely outside/inside the modulus */ |
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45 c->dp[b / DIGIT_BIT] &= |
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46 (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1)); |
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47 mp_clamp (c); |
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48 return MP_OKAY; |
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49 } |
