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annotate libtommath/bn_mp_div_2d.c @ 1673:e0871128e61f

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CBC mode cleanup (#95) * Fix CBC mode can't be fully disabled * Fix CBC mode can't be the only mode
author Vladislav Grishenko <themiron@users.noreply.github.com>
date Mon, 25 May 2020 20:55:13 +0500
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_MP_DIV_2D_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 /* shift right by a certain bit count (store quotient in c, optional remainder in d) */
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16 int mp_div_2d(const mp_int *a, int b, mp_int *c, mp_int *d)
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17 {
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18 mp_digit D, r, rr;
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19 int x, res;
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20
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21 /* if the shift count is <= 0 then we do no work */
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22 if (b <= 0) {
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23 res = mp_copy(a, c);
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24 if (d != NULL) {
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25 mp_zero(d);
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26 }
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27 return res;
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28 }
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29
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30 /* copy */
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31 if ((res = mp_copy(a, c)) != MP_OKAY) {
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32 return res;
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33 }
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34 /* 'a' should not be used after here - it might be the same as d */
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35
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36 /* get the remainder */
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37 if (d != NULL) {
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38 if ((res = mp_mod_2d(a, b, d)) != MP_OKAY) {
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39 return res;
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40 }
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41 }
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42
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43 /* shift by as many digits in the bit count */
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44 if (b >= DIGIT_BIT) {
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45 mp_rshd(c, b / DIGIT_BIT);
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46 }
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47
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48 /* shift any bit count < DIGIT_BIT */
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49 D = (mp_digit)(b % DIGIT_BIT);
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50 if (D != 0u) {
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51 mp_digit *tmpc, mask, shift;
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52
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53 /* mask */
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54 mask = ((mp_digit)1 << D) - 1uL;
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55
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56 /* shift for lsb */
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57 shift = (mp_digit)DIGIT_BIT - D;
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58
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59 /* alias */
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60 tmpc = c->dp + (c->used - 1);
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61
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62 /* carry */
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63 r = 0;
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64 for (x = c->used - 1; x >= 0; x--) {
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65 /* get the lower bits of this word in a temp */
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66 rr = *tmpc & mask;
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67
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68 /* shift the current word and mix in the carry bits from the previous word */
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69 *tmpc = (*tmpc >> D) | (r << shift);
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70 --tmpc;
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71
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72 /* set the carry to the carry bits of the current word found above */
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73 r = rr;
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74 }
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75 }
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76 mp_clamp(c);
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77 return MP_OKAY;
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78 }
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79 #endif
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80
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81 /* ref: HEAD -> master, tag: v1.1.0 */
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82 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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83 /* commit time: 2019-01-28 20:32:32 +0100 */