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annotate libtommath/bn_mp_sub.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
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1 #include "tommath_private.h"
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2 #ifdef BN_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 /* high level subtraction (handles signs) */
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16 int mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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17 {
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18 int sa, sb, res;
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19
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20 sa = a->sign;
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21 sb = b->sign;
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22
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23 if (sa != sb) {
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24 /* subtract a negative from a positive, OR */
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25 /* subtract a positive from a negative. */
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26 /* In either case, ADD their magnitudes, */
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27 /* and use the sign of the first number. */
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28 c->sign = sa;
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29 res = s_mp_add(a, b, c);
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30 } else {
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31 /* subtract a positive from a positive, OR */
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32 /* subtract a negative from a negative. */
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33 /* First, take the difference between their */
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34 /* magnitudes, then... */
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35 if (mp_cmp_mag(a, b) != MP_LT) {
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36 /* Copy the sign from the first */
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37 c->sign = sa;
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38 /* The first has a larger or equal magnitude */
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39 res = s_mp_sub(a, b, c);
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40 } else {
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41 /* The result has the *opposite* sign from */
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42 /* the first number. */
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43 c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
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44 /* The second has a larger magnitude */
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45 res = s_mp_sub(b, a, c);
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46 }
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47 }
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48 return res;
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49 }
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50
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51 #endif
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52
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53 /* ref: HEAD -> master, tag: v1.1.0 */
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54 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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55 /* commit time: 2019-01-28 20:32:32 +0100 */