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