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annotate libtommath/bn_mp_expt_d_ex.c @ 1674:ba6fc7afe1c5

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author Matt Johnston <matt@ucc.asn.au>
date Mon, 06 Apr 2020 23:18:26 +0800
parents f52919ffd3b1
children
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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 */