Mercurial > dropbear

annotate libtommath/bn_s_mp_add.c @ 1761:4b984c42372d DROPBEAR_2020.81

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Changelog for 2020.81
author Matt Johnston <matt@ucc.asn.au>
date Thu, 29 Oct 2020 21:35:50 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_ADD_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
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4 /* SPDX-License-Identifier: Unlicense */
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6 /* low level addition, based on HAC pp.594, Algorithm 14.7 */
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7 mp_err s_mp_add(const mp_int *a, const mp_int *b, mp_int *c)
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8 {
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9 const mp_int *x;
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10 mp_err err;
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11 int olduse, min, max;
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12
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13 /* find sizes, we let |a| <= |b| which means we have to sort
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14 * them. "x" will point to the input with the most digits
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15 */
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16 if (a->used > b->used) {
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17 min = b->used;
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18 max = a->used;
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19 x = a;
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20 } else {
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21 min = a->used;
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22 max = b->used;
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23 x = b;
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24 }
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26 /* init result */
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27 if (c->alloc < (max + 1)) {
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28 if ((err = mp_grow(c, max + 1)) != MP_OKAY) {
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29 return err;
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30 }
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31 }
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32
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33 /* get old used digit count and set new one */
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34 olduse = c->used;
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35 c->used = max + 1;
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36
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37 {
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38 mp_digit u, *tmpa, *tmpb, *tmpc;
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39 int i;
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41 /* alias for digit pointers */
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42
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43 /* first input */
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44 tmpa = a->dp;
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45
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46 /* second input */
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47 tmpb = b->dp;
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49 /* destination */
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50 tmpc = c->dp;
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51
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52 /* zero the carry */
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53 u = 0;
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54 for (i = 0; i < min; i++) {
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55 /* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
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56 *tmpc = *tmpa++ + *tmpb++ + u;
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57
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58 /* U = carry bit of T[i] */
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59 u = *tmpc >> (mp_digit)MP_DIGIT_BIT;
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60
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61 /* take away carry bit from T[i] */
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62 *tmpc++ &= MP_MASK;
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63 }
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64
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65 /* now copy higher words if any, that is in A+B
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66 * if A or B has more digits add those in
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67 */
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68 if (min != max) {
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69 for (; i < max; i++) {
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70 /* T[i] = X[i] + U */
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71 *tmpc = x->dp[i] + u;
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72
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73 /* U = carry bit of T[i] */
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74 u = *tmpc >> (mp_digit)MP_DIGIT_BIT;
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75
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76 /* take away carry bit from T[i] */
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77 *tmpc++ &= MP_MASK;
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78 }
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79 }
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80
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81 /* add carry */
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82 *tmpc++ = u;
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83
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84 /* clear digits above oldused */
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85 MP_ZERO_DIGITS(tmpc, olduse - c->used);
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86 }
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87
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88 mp_clamp(c);
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89 return MP_OKAY;
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90 }
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91 #endif