i31_modpow.c

/*
 * Copyright (c) 2016 Thomas Pornin <[email protected]>
 *
 * Permission is hereby granted, free of charge, to any person obtaining 
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be 
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include "inner.h"

/* see inner.h */
void
br_i31_modpow(uint32_t *x,
	const unsigned char *e, size_t elen,
	const uint32_t *m, uint32_t m0i, uint32_t *t1, uint32_t *t2)
{
	size_t mlen;
	uint32_t k;

	/*
	 * 'mlen' is the length of m[] expressed in bytes (including
	 * the "bit length" first field).
	 */
	mlen = ((m[0] + 63) >> 5) * sizeof m[0];

	/*
	 * Throughout the algorithm:
	 * -- t1[] is in Montgomery representation; it contains x, x^2,
	 * x^4, x^8...
	 * -- The result is accumulated, in normal representation, in
	 * the x[] array.
	 * -- t2[] is used as destination buffer for each multiplication.
	 *
	 * Note that there is no need to call br_i32_from_monty().
	 */
	memcpy(t1, x, mlen);
	br_i31_to_monty(t1, m);
	br_i31_zero(x, m[0]);
	x[1] = 1;
	for (k = 0; k < ((uint32_t)elen << 3); k ++) {
		uint32_t ctl;

		ctl = (e[elen - 1 - (k >> 3)] >> (k & 7)) & 1;
		br_i31_montymul(t2, x, t1, m, m0i);
		CCOPY(ctl, x, t2, mlen);
		br_i31_montymul(t2, t1, t1, m, m0i);
		memcpy(t1, t2, mlen);
	}
}