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142
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1 #include <tommath.h> |
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2 #ifdef BN_MP_REDUCE_2K_C |
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2
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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 /* reduces a modulo n where n is of the form 2**p - d */ |
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19 int |
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20 mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d) |
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21 { |
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22 mp_int q; |
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23 int p, res; |
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24 |
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25 if ((res = mp_init(&q)) != MP_OKAY) { |
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26 return res; |
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27 } |
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28 |
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29 p = mp_count_bits(n); |
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30 top: |
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31 /* q = a/2**p, a = a mod 2**p */ |
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32 if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) { |
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33 goto ERR; |
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34 } |
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35 |
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36 if (d != 1) { |
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37 /* q = q * d */ |
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38 if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) { |
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39 goto ERR; |
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40 } |
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41 } |
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42 |
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43 /* a = a + q */ |
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44 if ((res = s_mp_add(a, &q, a)) != MP_OKAY) { |
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45 goto ERR; |
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46 } |
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47 |
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48 if (mp_cmp_mag(a, n) != MP_LT) { |
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49 s_mp_sub(a, n, a); |
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50 goto top; |
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51 } |
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52 |
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53 ERR: |
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54 mp_clear(&q); |
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55 return res; |
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56 } |
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57 |
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142
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58 #endif |
