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// SPDX-License-Identifier: BSD-2-Clause
/*
* Copyright (c) 2014, STMicroelectronics International N.V.
*/
#include "mpa.h"
#define swp(a, b) do { \
mpanum *tmp = *a; \
*a = *b; \
*b = tmp; \
} while (0)
/*------------------------------------------------------------
*
* mpa_exp_mod
*
* Calculates dest = op1 ^ op2 mod n
*
* This function uses the Montgomery ladder concept as proposed by Marc Joye and
* Sun-Ming Yen, which makes the function more resistant to timing attacks.
*/
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 tmp_a;
mpanum xtilde;
mpanum tmp_xtilde;
mpanum *ptr_a;
mpanum *ptr_tmp_a;
mpanum *ptr_xtilde;
mpanum *ptr_tmp_xtilde;
int idx;
mpa_alloc_static_temp_var(&A, pool);
mpa_alloc_static_temp_var(&tmp_a, pool);
mpa_alloc_static_temp_var(&xtilde, pool);
mpa_alloc_static_temp_var(&tmp_xtilde, pool);
/*
* Transform the base (op1) into Montgomery space. Use internal version
* since xtilde is big enough.
*/
__mpa_montgomery_mul(xtilde, op1, r2_modn, n, n_inv);
mpa_copy(A, r_modn);
ptr_a = &A;
ptr_tmp_a = &tmp_a;
ptr_xtilde = &xtilde;
ptr_tmp_xtilde = &tmp_xtilde;
__mpa_set_unused_digits_to_zero(A);
__mpa_set_unused_digits_to_zero(xtilde);
for (idx = mpa_highest_bit_index(op2); idx >= 0; idx--) {
if (mpa_get_bit(op2, idx) == 0) {
/* x' = A*x' */
__mpa_montgomery_mul(*ptr_tmp_xtilde, *ptr_a,
*ptr_xtilde, n, n_inv);
/* A = A^2 */
__mpa_montgomery_mul(*ptr_tmp_a, *ptr_a, *ptr_a, n,
n_inv);
} else {
/* A = A*x' */
__mpa_montgomery_mul(*ptr_tmp_a, *ptr_a, *ptr_xtilde, n,
n_inv);
/* x' = x'^2 */
__mpa_montgomery_mul(*ptr_tmp_xtilde, *ptr_xtilde,
*ptr_xtilde, n, n_inv);
}
/*
* The simple reason for swapping here is to avoid copy
* intermediate results, instead we're just moving the pointers.
*/
swp(&ptr_tmp_a, &ptr_a);
swp(&ptr_tmp_xtilde, &ptr_xtilde);
}
/* Transform back from Montgomery space */
__mpa_montgomery_mul(*ptr_tmp_a, (const mpanum)&const_one, *ptr_a,
n, n_inv);
mpa_copy(dest, *ptr_tmp_a);
mpa_free_static_temp_var(&A, pool);
mpa_free_static_temp_var(&tmp_a, pool);
mpa_free_static_temp_var(&xtilde, pool);
mpa_free_static_temp_var(&tmp_xtilde, pool);
}
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