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annotate libtommath/bn_mp_exptmod.c @ 1827:8a78cc13eb30

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Fix some outdated comments
author Matt Johnston <matt@ucc.asn.au>
date Mon, 11 Oct 2021 15:14:46 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_EXPTMOD_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 /* this is a shell function that calls either the normal or Montgomery
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7 * exptmod functions. Originally the call to the montgomery code was
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8 * embedded in the normal function but that wasted alot of stack space
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9 * for nothing (since 99% of the time the Montgomery code would be called)
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10 */
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11 mp_err mp_exptmod(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y)
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12 {
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13 int dr;
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14
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15 /* modulus P must be positive */
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16 if (P->sign == MP_NEG) {
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17 return MP_VAL;
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18 }
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19
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20 /* if exponent X is negative we have to recurse */
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21 if (X->sign == MP_NEG) {
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22 mp_int tmpG, tmpX;
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23 mp_err err;
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24
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25 if (!MP_HAS(MP_INVMOD)) {
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26 return MP_VAL;
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27 }
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28
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29 if ((err = mp_init_multi(&tmpG, &tmpX, NULL)) != MP_OKAY) {
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30 return err;
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31 }
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32
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33 /* first compute 1/G mod P */
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34 if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
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35 goto LBL_ERR;
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36 }
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37
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38 /* now get |X| */
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39 if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
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40 goto LBL_ERR;
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41 }
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42
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43 /* and now compute (1/G)**|X| instead of G**X [X < 0] */
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44 err = mp_exptmod(&tmpG, &tmpX, P, Y);
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45 LBL_ERR:
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46 mp_clear_multi(&tmpG, &tmpX, NULL);
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47 return err;
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48 }
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49
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50 /* modified diminished radix reduction */
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51 if (MP_HAS(MP_REDUCE_IS_2K_L) && MP_HAS(MP_REDUCE_2K_L) && MP_HAS(S_MP_EXPTMOD) &&
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52 (mp_reduce_is_2k_l(P) == MP_YES)) {
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53 return s_mp_exptmod(G, X, P, Y, 1);
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54 }
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55
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56 /* is it a DR modulus? default to no */
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57 dr = (MP_HAS(MP_DR_IS_MODULUS) && (mp_dr_is_modulus(P) == MP_YES)) ? 1 : 0;
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58
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59 /* if not, is it a unrestricted DR modulus? */
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60 if (MP_HAS(MP_REDUCE_IS_2K) && (dr == 0)) {
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61 dr = (mp_reduce_is_2k(P) == MP_YES) ? 2 : 0;
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62 }
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63
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64 /* if the modulus is odd or dr != 0 use the montgomery method */
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65 if (MP_HAS(S_MP_EXPTMOD_FAST) && (MP_IS_ODD(P) || (dr != 0))) {
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66 return s_mp_exptmod_fast(G, X, P, Y, dr);
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67 } else if (MP_HAS(S_MP_EXPTMOD)) {
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68 /* otherwise use the generic Barrett reduction technique */
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69 return s_mp_exptmod(G, X, P, Y, 0);
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70 } else {
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71 /* no exptmod for evens */
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72 return MP_VAL;
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73 }
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74 }
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75
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76 #endif