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 */
28uint32_t
29br_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