Mercurial > dropbear
annotate libtommath/bn_mp_montgomery_reduce.c @ 1663:c795520269f9
Fallback for key gen without hard link support (#89)
Add a non-atomic fallback for key generation on platforms where link()
is not permitted (such as most stock Android installs) or on filesystems
without hard link support (such as FAT).
author | Matt Robinson <git@nerdoftheherd.com> |
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date | Sat, 14 Mar 2020 14:37:35 +0000 |
parents | f52919ffd3b1 |
children | 1051e4eea25a |
rev | line source |
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1 #include "tommath_private.h" |
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2 #ifdef BN_MP_MONTGOMERY_REDUCE_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 * SPDX-License-Identifier: Unlicense |
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13 */ |
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14 |
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15 /* computes xR**-1 == x (mod N) via Montgomery Reduction */ |
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16 int mp_montgomery_reduce(mp_int *x, const mp_int *n, mp_digit rho) |
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17 { |
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18 int ix, res, digs; |
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19 mp_digit mu; |
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20 |
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21 /* can the fast reduction [comba] method be used? |
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22 * |
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23 * Note that unlike in mul you're safely allowed *less* |
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24 * than the available columns [255 per default] since carries |
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25 * are fixed up in the inner loop. |
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26 */ |
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27 digs = (n->used * 2) + 1; |
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28 if ((digs < (int)MP_WARRAY) && |
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29 (x->used <= (int)MP_WARRAY) && |
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30 (n->used < |
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31 (int)(1u << (((size_t)CHAR_BIT * sizeof(mp_word)) - (2u * (size_t)DIGIT_BIT))))) { |
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32 return fast_mp_montgomery_reduce(x, n, rho); |
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33 } |
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34 |
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35 /* grow the input as required */ |
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36 if (x->alloc < digs) { |
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37 if ((res = mp_grow(x, digs)) != MP_OKAY) { |
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38 return res; |
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39 } |
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40 } |
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41 x->used = digs; |
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42 |
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43 for (ix = 0; ix < n->used; ix++) { |
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44 /* mu = ai * rho mod b |
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45 * |
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46 * The value of rho must be precalculated via |
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47 * montgomery_setup() such that |
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48 * it equals -1/n0 mod b this allows the |
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49 * following inner loop to reduce the |
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50 * input one digit at a time |
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51 */ |
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52 mu = (mp_digit)(((mp_word)x->dp[ix] * (mp_word)rho) & MP_MASK); |
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53 |
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54 /* a = a + mu * m * b**i */ |
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55 { |
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56 int iy; |
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57 mp_digit *tmpn, *tmpx, u; |
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58 mp_word r; |
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59 |
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60 /* alias for digits of the modulus */ |
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61 tmpn = n->dp; |
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62 |
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63 /* alias for the digits of x [the input] */ |
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64 tmpx = x->dp + ix; |
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65 |
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66 /* set the carry to zero */ |
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67 u = 0; |
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68 |
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69 /* Multiply and add in place */ |
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70 for (iy = 0; iy < n->used; iy++) { |
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71 /* compute product and sum */ |
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72 r = ((mp_word)mu * (mp_word)*tmpn++) + |
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73 (mp_word)u + (mp_word)*tmpx; |
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74 |
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75 /* get carry */ |
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76 u = (mp_digit)(r >> (mp_word)DIGIT_BIT); |
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77 |
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78 /* fix digit */ |
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79 *tmpx++ = (mp_digit)(r & (mp_word)MP_MASK); |
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80 } |
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81 /* At this point the ix'th digit of x should be zero */ |
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82 |
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83 |
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84 /* propagate carries upwards as required*/ |
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85 while (u != 0u) { |
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86 *tmpx += u; |
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87 u = *tmpx >> DIGIT_BIT; |
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88 *tmpx++ &= MP_MASK; |
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89 } |
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90 } |
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91 } |
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92 |
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93 /* at this point the n.used'th least |
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94 * significant digits of x are all zero |
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95 * which means we can shift x to the |
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96 * right by n.used digits and the |
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97 * residue is unchanged. |
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98 */ |
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99 |
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100 /* x = x/b**n.used */ |
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101 mp_clamp(x); |
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102 mp_rshd(x, n->used); |
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103 |
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104 /* if x >= n then x = x - n */ |
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105 if (mp_cmp_mag(x, n) != MP_LT) { |
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106 return s_mp_sub(x, n, x); |
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107 } |
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108 |
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109 return MP_OKAY; |
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110 } |
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111 #endif |
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112 |
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113 /* ref: HEAD -> master, tag: v1.1.0 */ |
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114 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */ |
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115 /* commit time: 2019-01-28 20:32:32 +0100 */ |