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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 /* Greatest Common Divisor using the binary method */ |
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18 int mp_gcd (mp_int * a, mp_int * b, mp_int * c) |
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19 { |
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20 mp_int u, v; |
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21 int k, u_lsb, v_lsb, res; |
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22 |
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23 /* either zero than gcd is the largest */ |
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24 if (mp_iszero (a) == 1 && mp_iszero (b) == 0) { |
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25 return mp_abs (b, c); |
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26 } |
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27 if (mp_iszero (a) == 0 && mp_iszero (b) == 1) { |
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28 return mp_abs (a, c); |
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29 } |
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30 |
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31 /* optimized. At this point if a == 0 then |
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32 * b must equal zero too |
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33 */ |
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34 if (mp_iszero (a) == 1) { |
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35 mp_zero(c); |
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36 return MP_OKAY; |
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37 } |
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38 |
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39 /* get copies of a and b we can modify */ |
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40 if ((res = mp_init_copy (&u, a)) != MP_OKAY) { |
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41 return res; |
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42 } |
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43 |
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44 if ((res = mp_init_copy (&v, b)) != MP_OKAY) { |
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45 goto __U; |
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46 } |
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47 |
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48 /* must be positive for the remainder of the algorithm */ |
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49 u.sign = v.sign = MP_ZPOS; |
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50 |
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51 /* B1. Find the common power of two for u and v */ |
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52 u_lsb = mp_cnt_lsb(&u); |
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53 v_lsb = mp_cnt_lsb(&v); |
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54 k = MIN(u_lsb, v_lsb); |
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55 |
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56 if (k > 0) { |
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57 /* divide the power of two out */ |
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58 if ((res = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) { |
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59 goto __V; |
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60 } |
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61 |
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62 if ((res = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) { |
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63 goto __V; |
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64 } |
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65 } |
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66 |
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67 /* divide any remaining factors of two out */ |
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68 if (u_lsb != k) { |
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69 if ((res = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) { |
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70 goto __V; |
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71 } |
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72 } |
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73 |
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74 if (v_lsb != k) { |
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75 if ((res = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) { |
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76 goto __V; |
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77 } |
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78 } |
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79 |
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80 while (mp_iszero(&v) == 0) { |
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81 /* make sure v is the largest */ |
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82 if (mp_cmp_mag(&u, &v) == MP_GT) { |
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83 /* swap u and v to make sure v is >= u */ |
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84 mp_exch(&u, &v); |
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85 } |
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86 |
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87 /* subtract smallest from largest */ |
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88 if ((res = s_mp_sub(&v, &u, &v)) != MP_OKAY) { |
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89 goto __V; |
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90 } |
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91 |
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92 /* Divide out all factors of two */ |
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93 if ((res = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) { |
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94 goto __V; |
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95 } |
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96 } |
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97 |
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98 /* multiply by 2**k which we divided out at the beginning */ |
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99 if ((res = mp_mul_2d (&u, k, c)) != MP_OKAY) { |
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100 goto __V; |
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101 } |
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102 c->sign = MP_ZPOS; |
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103 res = MP_OKAY; |
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104 __V:mp_clear (&u); |
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105 __U:mp_clear (&v); |
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106 return res; |
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107 } |
