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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 static int s_is_power_of_two(mp_digit b, int *p) |
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18 { |
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19 int x; |
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20 |
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21 for (x = 1; x < DIGIT_BIT; x++) { |
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22 if (b == (((mp_digit)1)<<x)) { |
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23 *p = x; |
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24 return 1; |
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25 } |
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26 } |
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27 return 0; |
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28 } |
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29 |
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30 /* single digit division (based on routine from MPI) */ |
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31 int mp_div_d (mp_int * a, mp_digit b, mp_int * c, mp_digit * d) |
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32 { |
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33 mp_int q; |
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34 mp_word w; |
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35 mp_digit t; |
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36 int res, ix; |
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37 |
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38 /* cannot divide by zero */ |
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39 if (b == 0) { |
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40 return MP_VAL; |
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41 } |
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42 |
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43 /* quick outs */ |
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44 if (b == 1 || mp_iszero(a) == 1) { |
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45 if (d != NULL) { |
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46 *d = 0; |
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47 } |
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48 if (c != NULL) { |
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49 return mp_copy(a, c); |
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50 } |
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51 return MP_OKAY; |
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52 } |
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53 |
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54 /* power of two ? */ |
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55 if (s_is_power_of_two(b, &ix) == 1) { |
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56 if (d != NULL) { |
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57 *d = a->dp[0] & ((1<<ix) - 1); |
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58 } |
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59 if (c != NULL) { |
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60 return mp_div_2d(a, ix, c, NULL); |
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61 } |
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62 return MP_OKAY; |
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63 } |
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64 |
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65 /* three? */ |
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66 if (b == 3) { |
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67 return mp_div_3(a, c, d); |
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68 } |
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69 |
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70 /* no easy answer [c'est la vie]. Just division */ |
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71 if ((res = mp_init_size(&q, a->used)) != MP_OKAY) { |
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72 return res; |
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73 } |
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74 |
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75 q.used = a->used; |
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76 q.sign = a->sign; |
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77 w = 0; |
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78 for (ix = a->used - 1; ix >= 0; ix--) { |
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79 w = (w << ((mp_word)DIGIT_BIT)) | ((mp_word)a->dp[ix]); |
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80 |
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81 if (w >= b) { |
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82 t = (mp_digit)(w / b); |
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83 w -= ((mp_word)t) * ((mp_word)b); |
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84 } else { |
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85 t = 0; |
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86 } |
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87 q.dp[ix] = (mp_digit)t; |
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88 } |
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89 |
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90 if (d != NULL) { |
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91 *d = (mp_digit)w; |
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92 } |
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93 |
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94 if (c != NULL) { |
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95 mp_clamp(&q); |
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96 mp_exch(&q, c); |
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97 } |
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98 mp_clear(&q); |
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99 |
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100 return res; |
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101 } |
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102 |
