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annotate libtommath/bn_s_mp_sub.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:
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_SUB_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 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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16 int s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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17 {
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18 int olduse, res, min, max;
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20 /* find sizes */
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21 min = b->used;
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22 max = a->used;
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24 /* init result */
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25 if (c->alloc < max) {
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26 if ((res = mp_grow(c, max)) != MP_OKAY) {
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27 return res;
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28 }
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29 }
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30 olduse = c->used;
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31 c->used = max;
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32
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33 {
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34 mp_digit u, *tmpa, *tmpb, *tmpc;
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35 int i;
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37 /* alias for digit pointers */
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38 tmpa = a->dp;
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39 tmpb = b->dp;
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40 tmpc = c->dp;
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41
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42 /* set carry to zero */
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43 u = 0;
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44 for (i = 0; i < min; i++) {
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45 /* T[i] = A[i] - B[i] - U */
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46 *tmpc = (*tmpa++ - *tmpb++) - u;
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48 /* U = carry bit of T[i]
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49 * Note this saves performing an AND operation since
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50 * if a carry does occur it will propagate all the way to the
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51 * MSB. As a result a single shift is enough to get the carry
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52 */
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53 u = *tmpc >> (((size_t)CHAR_BIT * sizeof(mp_digit)) - 1u);
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54
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55 /* Clear carry from T[i] */
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56 *tmpc++ &= MP_MASK;
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57 }
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59 /* now copy higher words if any, e.g. if A has more digits than B */
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60 for (; i < max; i++) {
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61 /* T[i] = A[i] - U */
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62 *tmpc = *tmpa++ - u;
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63
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64 /* U = carry bit of T[i] */
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65 u = *tmpc >> (((size_t)CHAR_BIT * sizeof(mp_digit)) - 1u);
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67 /* Clear carry from T[i] */
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68 *tmpc++ &= MP_MASK;
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69 }
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71 /* clear digits above used (since we may not have grown result above) */
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72 for (i = c->used; i < olduse; i++) {
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73 *tmpc++ = 0;
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74 }
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75 }
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77 mp_clamp(c);
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78 return MP_OKAY;
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79 }
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81 #endif
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83 /* ref: HEAD -> master, tag: v1.1.0 */
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84 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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85 /* commit time: 2019-01-28 20:32:32 +0100 */