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annotate libtommath/bn_s_mp_mul_digs.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_S_MP_MUL_DIGS_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 /* multiplies |a| * |b| and only computes upto digs digits of result
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16 * HAC pp. 595, Algorithm 14.12 Modified so you can control how
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17 * many digits of output are created.
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18 */
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19 int s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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20 {
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21 mp_int t;
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22 int res, pa, pb, ix, iy;
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23 mp_digit u;
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24 mp_word r;
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25 mp_digit tmpx, *tmpt, *tmpy;
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26
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27 /* can we use the fast multiplier? */
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28 if ((digs < (int)MP_WARRAY) &&
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29 (MIN(a->used, b->used) <
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30 (int)(1u << (((size_t)CHAR_BIT * sizeof(mp_word)) - (2u * (size_t)DIGIT_BIT))))) {
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31 return fast_s_mp_mul_digs(a, b, c, digs);
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32 }
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33
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34 if ((res = mp_init_size(&t, digs)) != MP_OKAY) {
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35 return res;
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36 }
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37 t.used = digs;
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38
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39 /* compute the digits of the product directly */
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40 pa = a->used;
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41 for (ix = 0; ix < pa; ix++) {
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42 /* set the carry to zero */
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43 u = 0;
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45 /* limit ourselves to making digs digits of output */
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46 pb = MIN(b->used, digs - ix);
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47
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48 /* setup some aliases */
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49 /* copy of the digit from a used within the nested loop */
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50 tmpx = a->dp[ix];
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51
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52 /* an alias for the destination shifted ix places */
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53 tmpt = t.dp + ix;
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54
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55 /* an alias for the digits of b */
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56 tmpy = b->dp;
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57
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58 /* compute the columns of the output and propagate the carry */
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59 for (iy = 0; iy < pb; iy++) {
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60 /* compute the column as a mp_word */
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61 r = (mp_word)*tmpt +
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62 ((mp_word)tmpx * (mp_word)*tmpy++) +
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63 (mp_word)u;
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64
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65 /* the new column is the lower part of the result */
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66 *tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
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68 /* get the carry word from the result */
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69 u = (mp_digit)(r >> (mp_word)DIGIT_BIT);
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70 }
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71 /* set carry if it is placed below digs */
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72 if ((ix + iy) < digs) {
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73 *tmpt = u;
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74 }
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75 }
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77 mp_clamp(&t);
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78 mp_exch(&t, c);
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79
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80 mp_clear(&t);
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81 return MP_OKAY;
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82 }
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83 #endif
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84
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85 /* ref: HEAD -> master, tag: v1.1.0 */
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86 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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87 /* commit time: 2019-01-28 20:32:32 +0100 */