Mercurial > dropbear

annotate libtommath/bn_mp_exptmod.c @ 691:e698d1a9f428

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Some changes since 2012.55
author Matt Johnston <matt@ucc.asn.au>
date Fri, 22 Feb 2013 23:54:47 +0800
parents 5ff8218bcee9
children 60fc6476e044
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284
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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 */
389
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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
389
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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 $ */