1/*
2 * Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining
5 * a copy of this software and associated documentation files (the
6 * "Software"), to deal in the Software without restriction, including
7 * without limitation the rights to use, copy, modify, merge, publish,
8 * distribute, sublicense, and/or sell copies of the Software, and to
9 * permit persons to whom the Software is furnished to do so, subject to
10 * the following conditions:
11 *
12 * The above copyright notice and this permission notice shall be
13 * included in all copies or substantial portions of the Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
22 * SOFTWARE.
23 */
24
25#include "inner.h"
26
27/* see inner.h */
28void
29br_i32_modpow(uint32_t *x,
30	const unsigned char *e, size_t elen,
31	const uint32_t *m, uint32_t m0i, uint32_t *t1, uint32_t *t2)
32{
33	size_t mlen;
34	uint32_t k;
35
36	/*
37	 * 'mlen' is the length of m[] expressed in bytes (including
38	 * the "bit length" first field).
39	 */
40	mlen = ((m[0] + 63) >> 5) * sizeof m[0];
41
42	/*
43	 * Throughout the algorithm:
44	 * -- t1[] is in Montgomery representation; it contains x, x^2,
45	 * x^4, x^8...
46	 * -- The result is accumulated, in normal representation, in
47	 * the x[] array.
48	 * -- t2[] is used as destination buffer for each multiplication.
49	 *
50	 * Note that there is no need to call br_i32_from_monty().
51	 */
52	memcpy(t1, x, mlen);
53	br_i32_to_monty(t1, m);
54	br_i32_zero(x, m[0]);
55	x[1] = 1;
56	for (k = 0; k < ((uint32_t)elen << 3); k ++) {
57		uint32_t ctl;
58
59		ctl = (e[elen - 1 - (k >> 3)] >> (k & 7)) & 1;
60		br_i32_montymul(t2, x, t1, m, m0i);
61		CCOPY(ctl, x, t2, mlen);
62		br_i32_montymul(t2, t1, t1, m, m0i);
63		memcpy(t1, t2, mlen);
64	}
65}
66