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142
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1 #include <tommath.h> |
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2 #ifdef BN_MP_REDUCE_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 x mod m, assumes 0 < x < m**2, mu is |
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19 * precomputed via mp_reduce_setup. |
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20 * From HAC pp.604 Algorithm 14.42 |
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21 */ |
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22 int |
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23 mp_reduce (mp_int * x, mp_int * m, mp_int * mu) |
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24 { |
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25 mp_int q; |
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26 int res, um = m->used; |
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27 |
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28 /* q = x */ |
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29 if ((res = mp_init_copy (&q, x)) != MP_OKAY) { |
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30 return res; |
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31 } |
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32 |
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33 /* q1 = x / b**(k-1) */ |
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34 mp_rshd (&q, um - 1); |
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35 |
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36 /* according to HAC this optimization is ok */ |
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37 if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) { |
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38 if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) { |
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39 goto CLEANUP; |
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40 } |
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41 } else { |
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142
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42 #ifdef BN_S_MP_MUL_HIGH_DIGS_C |
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2
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43 if ((res = s_mp_mul_high_digs (&q, mu, &q, um - 1)) != MP_OKAY) { |
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44 goto CLEANUP; |
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45 } |
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142
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46 #elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C) |
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47 if ((res = fast_s_mp_mul_high_digs (&q, mu, &q, um - 1)) != MP_OKAY) { |
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48 goto CLEANUP; |
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49 } |
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50 #else |
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51 { |
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52 res = MP_VAL; |
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53 goto CLEANUP; |
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54 } |
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55 #endif |
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2
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56 } |
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57 |
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58 /* q3 = q2 / b**(k+1) */ |
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59 mp_rshd (&q, um + 1); |
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60 |
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61 /* x = x mod b**(k+1), quick (no division) */ |
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62 if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) { |
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63 goto CLEANUP; |
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64 } |
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65 |
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66 /* q = q * m mod b**(k+1), quick (no division) */ |
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67 if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) { |
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68 goto CLEANUP; |
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69 } |
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70 |
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71 /* x = x - q */ |
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72 if ((res = mp_sub (x, &q, x)) != MP_OKAY) { |
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73 goto CLEANUP; |
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74 } |
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75 |
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76 /* If x < 0, add b**(k+1) to it */ |
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77 if (mp_cmp_d (x, 0) == MP_LT) { |
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78 mp_set (&q, 1); |
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79 if ((res = mp_lshd (&q, um + 1)) != MP_OKAY) |
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80 goto CLEANUP; |
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81 if ((res = mp_add (x, &q, x)) != MP_OKAY) |
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82 goto CLEANUP; |
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83 } |
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84 |
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85 /* Back off if it's too big */ |
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86 while (mp_cmp (x, m) != MP_LT) { |
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87 if ((res = s_mp_sub (x, m, x)) != MP_OKAY) { |
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88 goto CLEANUP; |
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89 } |
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90 } |
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91 |
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92 CLEANUP: |
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93 mp_clear (&q); |
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94 |
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95 return res; |
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96 } |
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142
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97 #endif |
