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1 #include <tommath.h> |
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2 #ifdef BN_MP_EXPTMOD_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 |
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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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142
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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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142
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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 #ifdef BN_MP_DR_IS_MODULUS_C |
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69 /* is it a DR modulus? */ |
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70 dr = mp_dr_is_modulus(P); |
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142
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71 #else |
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72 dr = 0; |
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73 #endif |
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74 |
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75 #ifdef BN_MP_REDUCE_IS_2K_C |
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76 /* if not, is it a uDR modulus? */ |
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77 if (dr == 0) { |
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78 dr = mp_reduce_is_2k(P) << 1; |
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79 } |
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142
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80 #endif |
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81 |
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82 /* if the modulus is odd or dr != 0 use the fast method */ |
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83 #ifdef BN_MP_EXPTMOD_FAST_C |
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84 if (mp_isodd (P) == 1 || dr != 0) { |
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85 return mp_exptmod_fast (G, X, P, Y, dr); |
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86 } else { |
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142
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87 #endif |
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88 #ifdef BN_S_MP_EXPTMOD_C |
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89 /* otherwise use the generic Barrett reduction technique */ |
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90 return s_mp_exptmod (G, X, P, Y); |
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142
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91 #else |
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92 /* no exptmod for evens */ |
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93 return MP_VAL; |
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94 #endif |
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95 #ifdef BN_MP_EXPTMOD_FAST_C |
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96 } |
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142
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97 #endif |
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98 } |
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99 |
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142
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100 #endif |
