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annotate libtommath/bn_mp_reduce.c @ 1692:1051e4eea25a

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Update LibTomMath to 1.2.0 (#84) * update C files * update other files * update headers * update makefiles * remove mp_set/get_double() * use ltm 1.2.0 API * update ltm_desc * use bundled tommath if system-tommath is too old * XMALLOC etc. were changed to MP_MALLOC etc.
author Steffen Jaeckel <s@jaeckel.eu>
date Tue, 26 May 2020 17:36:47 +0200
parents f52919ffd3b1
children
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rev   line source
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f52919ffd3b1 update ltm to 1.1.0 and enable FIPS 186.4 compliant key-generation (#79)
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diff changeset
1 #include "tommath_private.h"
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2 #ifdef BN_MP_REDUCE_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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5
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6 /* reduces x mod m, assumes 0 < x < m**2, mu is
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7 * precomputed via mp_reduce_setup.
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8 * From HAC pp.604 Algorithm 14.42
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9 */
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10 mp_err mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
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11 {
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12 mp_int q;
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13 mp_err err;
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14 int um = m->used;
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15
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16 /* q = x */
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17 if ((err = mp_init_copy(&q, x)) != MP_OKAY) {
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18 return err;
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19 }
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20
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21 /* q1 = x / b**(k-1) */
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22 mp_rshd(&q, um - 1);
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23
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24 /* according to HAC this optimization is ok */
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25 if ((mp_digit)um > ((mp_digit)1 << (MP_DIGIT_BIT - 1))) {
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26 if ((err = mp_mul(&q, mu, &q)) != MP_OKAY) {
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27 goto CLEANUP;
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28 }
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29 } else if (MP_HAS(S_MP_MUL_HIGH_DIGS)) {
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30 if ((err = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
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31 goto CLEANUP;
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32 }
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33 } else if (MP_HAS(S_MP_MUL_HIGH_DIGS_FAST)) {
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34 if ((err = s_mp_mul_high_digs_fast(&q, mu, &q, um)) != MP_OKAY) {
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35 goto CLEANUP;
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36 }
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37 } else {
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38 err = MP_VAL;
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39 goto CLEANUP;
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40 }
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41
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42 /* q3 = q2 / b**(k+1) */
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43 mp_rshd(&q, um + 1);
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44
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45 /* x = x mod b**(k+1), quick (no division) */
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46 if ((err = mp_mod_2d(x, MP_DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
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47 goto CLEANUP;
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48 }
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49
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50 /* q = q * m mod b**(k+1), quick (no division) */
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51 if ((err = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
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52 goto CLEANUP;
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53 }
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54
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55 /* x = x - q */
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56 if ((err = mp_sub(x, &q, x)) != MP_OKAY) {
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57 goto CLEANUP;
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58 }
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59
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60 /* If x < 0, add b**(k+1) to it */
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61 if (mp_cmp_d(x, 0uL) == MP_LT) {
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62 mp_set(&q, 1uL);
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63 if ((err = mp_lshd(&q, um + 1)) != MP_OKAY) {
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64 goto CLEANUP;
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65 }
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66 if ((err = mp_add(x, &q, x)) != MP_OKAY) {
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67 goto CLEANUP;
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68 }
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69 }
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70
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71 /* Back off if it's too big */
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72 while (mp_cmp(x, m) != MP_LT) {
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73 if ((err = s_mp_sub(x, m, x)) != MP_OKAY) {
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74 goto CLEANUP;
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75 }
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76 }
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77
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78 CLEANUP:
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79 mp_clear(&q);
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80
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81 return err;
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82 }
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83 #endif