annotate libtommath/bn_mp_expt_d_ex.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>
date Sat, 14 Mar 2020 14:37:35 +0000
parents f52919ffd3b1
children
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rev   line source
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_EXPT_D_EX_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 /* calculate c = a**b using a square-multiply algorithm */
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16 int mp_expt_d_ex(const mp_int *a, mp_digit b, mp_int *c, int fast)
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17 {
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18 int res;
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19 unsigned int x;
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20
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21 mp_int g;
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22
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23 if ((res = mp_init_copy(&g, a)) != MP_OKAY) {
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24 return res;
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25 }
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26
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27 /* set initial result */
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28 mp_set(c, 1uL);
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29
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30 if (fast != 0) {
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31 while (b > 0u) {
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32 /* if the bit is set multiply */
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33 if ((b & 1u) != 0u) {
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34 if ((res = mp_mul(c, &g, c)) != MP_OKAY) {
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35 mp_clear(&g);
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36 return res;
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37 }
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38 }
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39
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40 /* square */
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41 if (b > 1u) {
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42 if ((res = mp_sqr(&g, &g)) != MP_OKAY) {
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43 mp_clear(&g);
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44 return res;
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45 }
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46 }
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47
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48 /* shift to next bit */
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49 b >>= 1;
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50 }
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51 } else {
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52 for (x = 0; x < (unsigned)DIGIT_BIT; x++) {
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53 /* square */
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54 if ((res = mp_sqr(c, c)) != MP_OKAY) {
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55 mp_clear(&g);
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56 return res;
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57 }
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58
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59 /* if the bit is set multiply */
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60 if ((b & ((mp_digit)1 << (DIGIT_BIT - 1))) != 0u) {
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61 if ((res = mp_mul(c, &g, c)) != MP_OKAY) {
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62 mp_clear(&g);
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63 return res;
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64 }
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65 }
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66
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67 /* shift to next bit */
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68 b <<= 1;
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69 }
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70 } /* if ... else */
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71
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72 mp_clear(&g);
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73 return MP_OKAY;
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74 }
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75 #endif
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76
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77 /* ref: HEAD -> master, tag: v1.1.0 */
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78 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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79 /* commit time: 2019-01-28 20:32:32 +0100 */