Mercurial > dropbear

annotate libtommath/bn_mp_exptmod.c @ 994:5c5ade336926

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Prefer stronger algorithms in algorithm negotiation. Prefer diffie-hellman-group14-sha1 (2048 bit) over diffie-hellman-group1-sha1 (1024 bit). Due to meet-in-the-middle attacks the effective key length of three key 3DES is 112 bits. AES is stronger and faster then 3DES. Prefer to delay the start of compression until after authentication has completed. This avoids exposing compression code to attacks from unauthenticated users. (github pull request #9)
author Fedor Brunner <fedor.brunner@azet.sk>
date Fri, 23 Jan 2015 23:00:25 +0800
parents 5ff8218bcee9
children 60fc6476e044
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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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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parents:
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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 $ */