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annotate libtommath/bn_mp_sub_d.c @ 1659:d32bcb5c557d

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Add Ed25519 support (#91) * Add support for Ed25519 as a public key type Ed25519 is a elliptic curve signature scheme that offers better security than ECDSA and DSA and good performance. It may be used for both user and host keys. OpenSSH key import and fuzzer are not supported yet. Initially inspired by Peter Szabo. * Add curve25519 and ed25519 fuzzers * Add import and export of Ed25519 keys
author Vladislav Grishenko <themiron@users.noreply.github.com>
date Wed, 11 Mar 2020 21:09:45 +0500
parents f52919ffd3b1
children 1051e4eea25a
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_SUB_D_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 /* single digit subtraction */
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16 int mp_sub_d(const mp_int *a, mp_digit b, mp_int *c)
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17 {
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18 mp_digit *tmpa, *tmpc, mu;
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19 int res, ix, oldused;
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20
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21 /* grow c as required */
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22 if (c->alloc < (a->used + 1)) {
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23 if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
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24 return res;
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25 }
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26 }
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27
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28 /* if a is negative just do an unsigned
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29 * addition [with fudged signs]
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30 */
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31 if (a->sign == MP_NEG) {
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32 mp_int a_ = *a;
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33 a_.sign = MP_ZPOS;
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34 res = mp_add_d(&a_, b, c);
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35 c->sign = MP_NEG;
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36
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37 /* clamp */
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38 mp_clamp(c);
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39
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40 return res;
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41 }
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42
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43 /* setup regs */
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44 oldused = c->used;
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45 tmpa = a->dp;
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46 tmpc = c->dp;
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47
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48 /* if a <= b simply fix the single digit */
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49 if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {
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50 if (a->used == 1) {
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51 *tmpc++ = b - *tmpa;
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52 } else {
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53 *tmpc++ = b;
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54 }
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55 ix = 1;
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56
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57 /* negative/1digit */
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58 c->sign = MP_NEG;
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59 c->used = 1;
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60 } else {
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61 /* positive/size */
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62 c->sign = MP_ZPOS;
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63 c->used = a->used;
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64
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65 /* subtract first digit */
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66 *tmpc = *tmpa++ - b;
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67 mu = *tmpc >> ((sizeof(mp_digit) * (size_t)CHAR_BIT) - 1u);
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68 *tmpc++ &= MP_MASK;
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70 /* handle rest of the digits */
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71 for (ix = 1; ix < a->used; ix++) {
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72 *tmpc = *tmpa++ - mu;
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73 mu = *tmpc >> ((sizeof(mp_digit) * (size_t)CHAR_BIT) - 1u);
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74 *tmpc++ &= MP_MASK;
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75 }
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76 }
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77
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78 /* zero excess digits */
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79 while (ix++ < oldused) {
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80 *tmpc++ = 0;
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81 }
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82 mp_clamp(c);
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83 return MP_OKAY;
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84 }
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86 #endif
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87
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88 /* ref: HEAD -> master, tag: v1.1.0 */
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89 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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90 /* commit time: 2019-01-28 20:32:32 +0100 */