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 /* see bearssl_rsa.h */
28 uint32_t
br_rsa_pkcs1_sig_unpad(const unsigned char * sig,size_t sig_len,const unsigned char * hash_oid,size_t hash_len,unsigned char * hash_out)29 br_rsa_pkcs1_sig_unpad(const unsigned char *sig, size_t sig_len,
30 const unsigned char *hash_oid, size_t hash_len,
31 unsigned char *hash_out)
32 {
33 static const unsigned char pad1[] = {
34 0x00, 0x01, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
35 };
36
37 unsigned char pad2[43];
38 size_t u, x2, x3, pad_len, zlen;
39
40 if (sig_len < 11) {
41 return 0;
42 }
43
44 /*
45 * Expected format:
46 * 00 01 FF ... FF 00 30 x1 30 x2 06 x3 OID [ 05 00 ] 04 x4 HASH
47 *
48 * with the following rules:
49 *
50 * -- Total length is that of the modulus and the signature
51 * (this was already verified by br_rsa_i31_public()).
52 *
53 * -- There are at least eight bytes of value 0xFF.
54 *
55 * -- x4 is equal to the hash length (hash_len).
56 *
57 * -- x3 is equal to the encoded OID value length (so x3 is the
58 * first byte of hash_oid[]).
59 *
60 * -- If the "05 00" is present, then x2 == x3 + 4; otherwise,
61 * x2 == x3 + 2.
62 *
63 * -- x1 == x2 + x4 + 4.
64 *
65 * So the total length after the last "FF" is either x3 + x4 + 11
66 * (with the "05 00") or x3 + x4 + 9 (without the "05 00").
67 */
68
69 /*
70 * Check the "00 01 FF .. FF 00" with at least eight 0xFF bytes.
71 * The comparison is valid because we made sure that the signature
72 * is at least 11 bytes long.
73 */
74 if (memcmp(sig, pad1, sizeof pad1) != 0) {
75 return 0;
76 }
77 for (u = sizeof pad1; u < sig_len; u ++) {
78 if (sig[u] != 0xFF) {
79 break;
80 }
81 }
82
83 /*
84 * Remaining length is sig_len - u bytes (including the 00 just
85 * after the last FF). This must be equal to one of the two
86 * possible values (depending on whether the "05 00" sequence is
87 * present or not).
88 */
89 if (hash_oid == NULL) {
90 if (sig_len - u != hash_len + 1 || sig[u] != 0x00) {
91 return 0;
92 }
93 } else {
94 x3 = hash_oid[0];
95 pad_len = x3 + 9;
96 memset(pad2, 0, pad_len);
97 zlen = sig_len - u - hash_len;
98 if (zlen == pad_len) {
99 x2 = x3 + 2;
100 } else if (zlen == pad_len + 2) {
101 x2 = x3 + 4;
102 pad_len = zlen;
103 pad2[pad_len - 4] = 0x05;
104 } else {
105 return 0;
106 }
107 pad2[1] = 0x30;
108 pad2[2] = x2 + hash_len + 4;
109 pad2[3] = 0x30;
110 pad2[4] = x2;
111 pad2[5] = 0x06;
112 memcpy(pad2 + 6, hash_oid, x3 + 1);
113 pad2[pad_len - 2] = 0x04;
114 pad2[pad_len - 1] = hash_len;
115 if (memcmp(pad2, sig + u, pad_len) != 0) {
116 return 0;
117 }
118 }
119 memcpy(hash_out, sig + sig_len - hash_len, hash_len);
120 return 1;
121 }
122