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_decode_reduce(uint32_t *x,
30	const void *src, size_t len, const uint32_t *m)
31{
32	uint32_t m_bitlen;
33	size_t mblen, k, q;
34	const unsigned char *buf;
35
36	m_bitlen = m[0];
37
38	/*
39	 * Special case for an invalid modulus.
40	 */
41	if (m_bitlen == 0) {
42		x[0] = 0;
43		return;
44	}
45
46	/*
47	 * Clear the destination.
48	 */
49	br_i32_zero(x, m_bitlen);
50
51	/*
52	 * First decode directly as many bytes as possible without
53	 * reduction, taking care to leave a number of bytes which
54	 * is a multiple of 4.
55	 */
56	mblen = (m_bitlen + 7) >> 3;
57	k = mblen - 1;
58
59	/*
60	 * Up to k bytes can be safely decoded.
61	 */
62	if (k >= len) {
63		br_i32_decode(x, src, len);
64		x[0] = m_bitlen;
65		return;
66	}
67
68	/*
69	 * We want to first inject some bytes with direct decoding,
70	 * then extra bytes by whole 32-bit words. First compute
71	 * the size that should be injected that way.
72	 */
73	buf = src;
74	q = (len - k + 3) & ~(size_t)3;
75
76	/*
77	 * It may happen that this is more than what we already have
78	 * (by at most 3 bytes). Such a case may happen only with
79	 * a very short modulus. In that case, we must process the first
80	 * bytes "manually".
81	 */
82	if (q > len) {
83		int i;
84		uint32_t w;
85
86		w = 0;
87		for (i = 0; i < 4; i ++) {
88			w <<= 8;
89			if (q <= len) {
90				w |= buf[len - q];
91			}
92			q --;
93		}
94		br_i32_muladd_small(x, w, m);
95	} else {
96		br_i32_decode(x, buf, len - q);
97		x[0] = m_bitlen;
98	}
99
100	/*
101	 * At that point, we have exactly q bytes to inject, and q is
102	 * a multiple of 4.
103	 */
104	for (k = len - q; k < len; k += 4) {
105		br_i32_muladd_small(x, br_dec32be(buf + k), m);
106	}
107}
108