Mercurial > dropbear

annotate libtommath/bn_mp_dr_reduce.c @ 1686:83d48276885b

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fix typo
author Matt Johnston <matt@ucc.asn.au>
date Tue, 26 May 2020 20:15:00 +0800
parents f52919ffd3b1
children 1051e4eea25a
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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 */