Fixed carry propagation bug in P-256 'm62' implementation (found by Auke Zeilstra...
[BearSSL] / src / rsa / rsa_i15_pub.c
1 /*
2 * Copyright (c) 2017 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 /*
28 * As a strict minimum, we need four buffers that can hold a
29 * modular integer.
30 */
31 #define TLEN (4 * (2 + ((BR_MAX_RSA_SIZE + 14) / 15)))
32
33 /* see bearssl_rsa.h */
34 uint32_t
35 br_rsa_i15_public(unsigned char *x, size_t xlen,
36 const br_rsa_public_key *pk)
37 {
38 const unsigned char *n;
39 size_t nlen;
40 uint16_t tmp[1 + TLEN];
41 uint16_t *m, *a, *t;
42 size_t fwlen;
43 long z;
44 uint16_t m0i;
45 uint32_t r;
46
47 /*
48 * Get the actual length of the modulus, and see if it fits within
49 * our stack buffer. We also check that the length of x[] is valid.
50 */
51 n = pk->n;
52 nlen = pk->nlen;
53 while (nlen > 0 && *n == 0) {
54 n ++;
55 nlen --;
56 }
57 if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
58 return 0;
59 }
60 z = (long)nlen << 3;
61 fwlen = 1;
62 while (z > 0) {
63 z -= 15;
64 fwlen ++;
65 }
66 /*
67 * Round up length to an even number.
68 */
69 fwlen += (fwlen & 1);
70
71 /*
72 * The modulus gets decoded into m[].
73 * The value to exponentiate goes into a[].
74 * The temporaries for modular exponentiations are in t[].
75 *
76 * We want the first value word of each integer to be aligned
77 * on a 32-bit boundary.
78 */
79 m = tmp;
80 if (((uintptr_t)m & 2) == 0) {
81 m ++;
82 }
83 a = m + fwlen;
84 t = m + 2 * fwlen;
85
86 /*
87 * Decode the modulus.
88 */
89 br_i15_decode(m, n, nlen);
90 m0i = br_i15_ninv15(m[1]);
91
92 /*
93 * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
94 * an odd integer.
95 */
96 r = m0i & 1;
97
98 /*
99 * Decode x[] into a[]; we also check that its value is proper.
100 */
101 r &= br_i15_decode_mod(a, x, xlen, m);
102
103 /*
104 * Compute the modular exponentiation.
105 */
106 br_i15_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen);
107
108 /*
109 * Encode the result.
110 */
111 br_i15_encode(x, xlen, a);
112 return r;
113 }