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 */
28 void
br_i32_decode_reduce(uint32_t * x,const void * src,size_t len,const uint32_t * m)29 br_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