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1 #include <tommath.h> |
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2 #ifdef BN_MP_EXPTMOD_C |
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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.com |
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16 */ |
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17 |
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18 |
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19 /* this is a shell function that calls either the normal or Montgomery |
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20 * exptmod functions. Originally the call to the montgomery code was |
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21 * embedded in the normal function but that wasted alot of stack space |
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22 * for nothing (since 99% of the time the Montgomery code would be called) |
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23 */ |
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24 int mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y) |
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25 { |
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26 int dr; |
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27 |
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28 /* modulus P must be positive */ |
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29 if (P->sign == MP_NEG) { |
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30 return MP_VAL; |
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31 } |
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32 |
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33 /* if exponent X is negative we have to recurse */ |
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34 if (X->sign == MP_NEG) { |
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35 #ifdef BN_MP_INVMOD_C |
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36 mp_int tmpG, tmpX; |
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37 int err; |
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38 |
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39 /* first compute 1/G mod P */ |
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40 if ((err = mp_init(&tmpG)) != MP_OKAY) { |
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41 return err; |
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42 } |
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43 if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) { |
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44 mp_clear(&tmpG); |
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45 return err; |
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46 } |
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47 |
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48 /* now get |X| */ |
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49 if ((err = mp_init(&tmpX)) != MP_OKAY) { |
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50 mp_clear(&tmpG); |
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51 return err; |
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52 } |
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53 if ((err = mp_abs(X, &tmpX)) != MP_OKAY) { |
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54 mp_clear_multi(&tmpG, &tmpX, NULL); |
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55 return err; |
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56 } |
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57 |
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58 /* and now compute (1/G)**|X| instead of G**X [X < 0] */ |
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59 err = mp_exptmod(&tmpG, &tmpX, P, Y); |
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60 mp_clear_multi(&tmpG, &tmpX, NULL); |
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61 return err; |
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62 #else |
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63 /* no invmod */ |
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64 return MP_VAL; |
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65 #endif |
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66 } |
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67 |
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68 /* modified diminished radix reduction */ |
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69 #if defined(BN_MP_REDUCE_IS_2K_L_C) && defined(BN_MP_REDUCE_2K_L_C) && defined(BN_S_MP_EXPTMOD_C) |
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70 if (mp_reduce_is_2k_l(P) == MP_YES) { |
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71 return s_mp_exptmod(G, X, P, Y, 1); |
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72 } |
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73 #endif |
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74 |
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75 #ifdef BN_MP_DR_IS_MODULUS_C |
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76 /* is it a DR modulus? */ |
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77 dr = mp_dr_is_modulus(P); |
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78 #else |
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79 /* default to no */ |
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80 dr = 0; |
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81 #endif |
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82 |
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83 #ifdef BN_MP_REDUCE_IS_2K_C |
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84 /* if not, is it a unrestricted DR modulus? */ |
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85 if (dr == 0) { |
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86 dr = mp_reduce_is_2k(P) << 1; |
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87 } |
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88 #endif |
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89 |
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90 /* if the modulus is odd or dr != 0 use the montgomery method */ |
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91 #ifdef BN_MP_EXPTMOD_FAST_C |
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92 if (mp_isodd (P) == 1 || dr != 0) { |
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93 return mp_exptmod_fast (G, X, P, Y, dr); |
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94 } else { |
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95 #endif |
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96 #ifdef BN_S_MP_EXPTMOD_C |
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97 /* otherwise use the generic Barrett reduction technique */ |
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98 return s_mp_exptmod (G, X, P, Y, 0); |
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99 #else |
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100 /* no exptmod for evens */ |
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101 return MP_VAL; |
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102 #endif |
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103 #ifdef BN_MP_EXPTMOD_FAST_C |
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104 } |
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105 #endif |
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106 } |
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107 |
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108 #endif |
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109 |
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110 /* $Source: /cvs/libtom/libtommath/bn_mp_exptmod.c,v $ */ |
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111 /* $Revision: 1.4 $ */ |
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112 /* $Date: 2006/03/31 14:18:44 $ */ |
