78 lines
2.6 KiB
C
78 lines
2.6 KiB
C
#include "amplify_tommath_private.h"
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#ifdef AMPLIFY_BN_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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/* Modifications Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. */
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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 alot 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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amplify_mp_err amplify_mp_exptmod(const amplify_mp_int *G, const amplify_mp_int *X, const amplify_mp_int *P, amplify_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 (P->sign == AMPLIFY_MP_NEG) {
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return AMPLIFY_MP_VAL;
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}
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/* if exponent X is negative we have to recurse */
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if (X->sign == AMPLIFY_MP_NEG) {
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amplify_mp_int tmpG, tmpX;
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amplify_mp_err err;
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if (!AMPLIFY_MP_HAS(MP_INVMOD)) {
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return AMPLIFY_MP_VAL;
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}
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if ((err = amplify_mp_init_multi(&tmpG, &tmpX, NULL)) != AMPLIFY_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 = amplify_mp_invmod(G, P, &tmpG)) != AMPLIFY_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 = amplify_mp_abs(X, &tmpX)) != AMPLIFY_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 = amplify_mp_exptmod(&tmpG, &tmpX, P, Y);
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LBL_ERR:
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amplify_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 (AMPLIFY_MP_HAS(MP_REDUCE_IS_2K_L) && AMPLIFY_MP_HAS(MP_REDUCE_2K_L) && AMPLIFY_MP_HAS(S_MP_EXPTMOD) &&
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(amplify_mp_reduce_is_2k_l(P) == AMPLIFY_MP_YES)) {
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return amplify_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 = (AMPLIFY_MP_HAS(MP_DR_IS_MODULUS) && (amplify_mp_dr_is_modulus(P) == AMPLIFY_MP_YES)) ? 1 : 0;
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/* if not, is it a unrestricted DR modulus? */
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if (AMPLIFY_MP_HAS(MP_REDUCE_IS_2K) && (dr == 0)) {
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dr = (amplify_mp_reduce_is_2k(P) == AMPLIFY_MP_YES) ? 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 (AMPLIFY_MP_HAS(S_MP_EXPTMOD_FAST) && (AMPLIFY_MP_IS_ODD(P) || (dr != 0))) {
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return amplify_s_mp_exptmod_fast(G, X, P, Y, dr);
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} else if (AMPLIFY_MP_HAS(S_MP_EXPTMOD)) {
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/* otherwise use the generic Barrett reduction technique */
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return amplify_s_mp_exptmod(G, X, P, Y, 0);
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} else {
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/* no exptmod for evens */
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return AMPLIFY_MP_VAL;
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}
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}
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#endif
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