79 lines
2.1 KiB
C
79 lines
2.1 KiB
C
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#include "tommath_private.h"
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#ifdef MP_EXPTMOD_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* this is a shell function that calls either the normal or Montgomery
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* exptmod functions. Originally the call to the montgomery code was
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* embedded in the normal function but that wasted a lot of stack space
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* for nothing (since 99% of the time the Montgomery code would be called)
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*/
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mp_err mp_exptmod(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y)
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{
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int dr;
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/* modulus P must be positive */
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if (mp_isneg(P)) {
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return MP_VAL;
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}
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/* if exponent X is negative we have to recurse */
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if (mp_isneg(X)) {
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mp_int tmpG, tmpX;
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mp_err err;
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if (!MP_HAS(MP_INVMOD)) {
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return MP_VAL;
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}
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if ((err = mp_init_multi(&tmpG, &tmpX, NULL)) != MP_OKAY) {
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return err;
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}
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/* first compute 1/G mod P */
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if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* now get |X| */
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if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* and now compute (1/G)**|X| instead of G**X [X < 0] */
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err = mp_exptmod(&tmpG, &tmpX, P, Y);
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LBL_ERR:
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mp_clear_multi(&tmpG, &tmpX, NULL);
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return err;
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}
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/* modified diminished radix reduction */
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if (MP_HAS(MP_REDUCE_IS_2K_L) && MP_HAS(MP_REDUCE_2K_L) && MP_HAS(S_MP_EXPTMOD) &&
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mp_reduce_is_2k_l(P)) {
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return s_mp_exptmod(G, X, P, Y, 1);
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}
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/* is it a DR modulus? default to no */
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dr = (MP_HAS(MP_DR_IS_MODULUS) && mp_dr_is_modulus(P)) ? 1 : 0;
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/* if not, is it a unrestricted DR modulus? */
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if (MP_HAS(MP_REDUCE_IS_2K) && (dr == 0)) {
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dr = (mp_reduce_is_2k(P)) ? 2 : 0;
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}
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/* if the modulus is odd or dr != 0 use the montgomery method */
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if (MP_HAS(S_MP_EXPTMOD_FAST) && (mp_isodd(P) || (dr != 0))) {
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return s_mp_exptmod_fast(G, X, P, Y, dr);
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}
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/* otherwise use the generic Barrett reduction technique */
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if (MP_HAS(S_MP_EXPTMOD)) {
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return s_mp_exptmod(G, X, P, Y, 0);
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}
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/* no exptmod for evens */
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return MP_VAL;
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}
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#endif
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