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/*
* Copyright (c) 2014, STMicroelectronics International N.V.
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
#include "mpa.h"
/*------------------------------------------------------------
*
* mpa_exp_mod
*
* Calculates dest = op1 ^ op2 mod n
*
*/
void mpa_exp_mod(mpanum dest,
const mpanum op1,
const mpanum op2,
const mpanum n,
const mpanum r_modn,
const mpanum r2_modn,
const mpa_word_t n_inv, mpa_scratch_mem pool)
{
mpanum A;
mpanum B;
mpanum xtilde;
mpanum *ptr_a;
mpanum *ptr_b;
mpanum *swapper;
int idx;
mpa_alloc_static_temp_var(&A, pool);
mpa_alloc_static_temp_var(&B, pool);
mpa_alloc_static_temp_var(&xtilde, pool);
/* transform to Montgomery space */
/* use internal version since xtidle is big enough */
__mpa_montgomery_mul(xtilde, op1, r2_modn, n, n_inv);
mpa_copy(A, r_modn);
ptr_a = &A;
ptr_b = &B;
__mpa_set_unused_digits_to_zero(A);
__mpa_set_unused_digits_to_zero(B);
for (idx = mpa_highest_bit_index(op2); idx >= 0; idx--) {
__mpa_montgomery_mul(*ptr_b, *ptr_a, *ptr_a, n, n_inv);
if (mpa_get_bit(op2, idx) == 1) {
__mpa_montgomery_mul(*ptr_a, *ptr_b, xtilde, n, n_inv);
} else {
swapper = ptr_a;
ptr_a = ptr_b;
ptr_b = swapper;
}
}
/* transform back form Montgomery space */
__mpa_montgomery_mul(*ptr_b, (const mpanum)&const_one, *ptr_a,
n, n_inv);
mpa_copy(dest, *ptr_b);
mpa_free_static_temp_var(&A, pool);
mpa_free_static_temp_var(&B, pool);
mpa_free_static_temp_var(&xtilde, pool);
}
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