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annotate libtommath/bn_mp_dr_reduce.c @ 1663:c795520269f9

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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>
date Sat, 14 Mar 2020 14:37:35 +0000
parents f52919ffd3b1
children 1051e4eea25a
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_DR_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 /* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
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16 *
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17 * Based on algorithm from the paper
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18 *
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19 * "Generating Efficient Primes for Discrete Log Cryptosystems"
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20 * Chae Hoon Lim, Pil Joong Lee,
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21 * POSTECH Information Research Laboratories
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22 *
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23 * The modulus must be of a special format [see manual]
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24 *
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25 * Has been modified to use algorithm 7.10 from the LTM book instead
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26 *
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27 * Input x must be in the range 0 <= x <= (n-1)**2
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28 */
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29 int mp_dr_reduce(mp_int *x, const mp_int *n, mp_digit k)
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30 {
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31 int err, i, m;
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32 mp_word r;
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33 mp_digit mu, *tmpx1, *tmpx2;
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34
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35 /* m = digits in modulus */
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36 m = n->used;
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37
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38 /* ensure that "x" has at least 2m digits */
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39 if (x->alloc < (m + m)) {
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40 if ((err = mp_grow(x, m + m)) != MP_OKAY) {
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41 return err;
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42 }
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43 }
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44
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45 /* top of loop, this is where the code resumes if
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46 * another reduction pass is required.
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47 */
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48 top:
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49 /* aliases for digits */
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50 /* alias for lower half of x */
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51 tmpx1 = x->dp;
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52
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53 /* alias for upper half of x, or x/B**m */
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54 tmpx2 = x->dp + m;
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55
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56 /* set carry to zero */
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57 mu = 0;
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58
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59 /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
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60 for (i = 0; i < m; i++) {
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61 r = ((mp_word)*tmpx2++ * (mp_word)k) + *tmpx1 + mu;
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62 *tmpx1++ = (mp_digit)(r & MP_MASK);
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63 mu = (mp_digit)(r >> ((mp_word)DIGIT_BIT));
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64 }
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65
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66 /* set final carry */
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67 *tmpx1++ = mu;
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68
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69 /* zero words above m */
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70 for (i = m + 1; i < x->used; i++) {
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71 *tmpx1++ = 0;
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72 }
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73
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74 /* clamp, sub and return */
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75 mp_clamp(x);
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76
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77 /* if x >= n then subtract and reduce again
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78 * Each successive "recursion" makes the input smaller and smaller.
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79 */
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80 if (mp_cmp_mag(x, n) != MP_LT) {
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81 if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
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82 return err;
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83 }
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84 goto top;
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85 }
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86 return MP_OKAY;
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87 }
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88 #endif
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89
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90 /* ref: HEAD -> master, tag: v1.1.0 */
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91 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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92 /* commit time: 2019-01-28 20:32:32 +0100 */