1/*
2 * Copyright (c) 2018 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 * Hash some data and XOR the result into the provided buffer. This is put
29 * as a separate function so that stack allocation of the hash function
30 * context is done only for the duration of the hash.
31 */
32static void
33xor_hash_data(const br_hash_class *dig, void *dst, const void *src, size_t len)
34{
35	br_hash_compat_context hc;
36	unsigned char tmp[64];
37	unsigned char *buf;
38	size_t u, hlen;
39
40	hc.vtable = dig;
41	dig->init(&hc.vtable);
42	dig->update(&hc.vtable, src, len);
43	dig->out(&hc.vtable, tmp);
44	buf = dst;
45	hlen = br_digest_size(dig);
46	for (u = 0; u < hlen; u ++) {
47		buf[u] ^= tmp[u];
48	}
49}
50
51/* see inner.h */
52uint32_t
53br_rsa_oaep_unpad(const br_hash_class *dig,
54	const void *label, size_t label_len,
55	void *data, size_t *len)
56{
57	size_t u, k, hlen;
58	unsigned char *buf;
59	uint32_t r, s, zlen;
60
61	hlen = br_digest_size(dig);
62	k = *len;
63	buf = data;
64
65	/*
66	 * There must be room for the padding.
67	 */
68	if (k < ((hlen << 1) + 2)) {
69		return 0;
70	}
71
72	/*
73	 * Unmask the seed, then the DB value.
74	 */
75	br_mgf1_xor(buf + 1, hlen, dig, buf + 1 + hlen, k - hlen - 1);
76	br_mgf1_xor(buf + 1 + hlen, k - hlen - 1, dig, buf + 1, hlen);
77
78	/*
79	 * Hash the label and XOR it with the value in the array; if
80	 * they are equal then these should yield only zeros.
81	 */
82	xor_hash_data(dig, buf + 1 + hlen, label, label_len);
83
84	/*
85	 * At that point, if the padding was correct, when we should
86	 * have: 0x00 || seed || 0x00 ... 0x00 0x01 || M
87	 * Padding is valid as long as:
88	 *  - There is at least hlen+1 leading bytes of value 0x00.
89	 *  - There is at least one non-zero byte.
90	 *  - The first (leftmost) non-zero byte has value 0x01.
91	 *
92	 * Ultimately, we may leak the resulting message length, i.e.
93	 * the position of the byte of value 0x01, but we must take care
94	 * to do so only if the number of zero bytes has been verified
95	 * to be at least hlen+1.
96	 *
97	 * The loop below counts the number of bytes of value 0x00, and
98	 * checks that the next byte has value 0x01, in constant-time.
99	 *
100	 *  - If the initial byte (before the seed) is not 0x00, then
101	 *    r and s are set to 0, and stay there.
102	 *  - Value r is 1 until the first non-zero byte is reached
103	 *    (after the seed); it switches to 0 at that point.
104	 *  - Value s is set to 1 if and only if the data encountered
105	 *    at the time of the transition of r from 1 to 0 has value
106	 *    exactly 0x01.
107	 *  - Value zlen counts the number of leading bytes of value zero
108	 *    (after the seed).
109	 */
110	r = 1 - ((buf[0] + 0xFF) >> 8);
111	s = 0;
112	zlen = 0;
113	for (u = hlen + 1; u < k; u ++) {
114		uint32_t w, nz;
115
116		w = buf[u];
117
118		/*
119		 * nz == 1 only for the first non-zero byte.
120		 */
121		nz = r & ((w + 0xFF) >> 8);
122		s |= nz & EQ(w, 0x01);
123		r &= NOT(nz);
124		zlen += r;
125	}
126
127	/*
128	 * Padding is correct only if s == 1, _and_ zlen >= hlen.
129	 */
130	s &= GE(zlen, (uint32_t)hlen);
131
132	/*
133	 * At that point, padding was verified, and we are now allowed
134	 * to make conditional jumps.
135	 */
136	if (s) {
137		size_t plen;
138
139		plen = 2 + hlen + zlen;
140		k -= plen;
141		memmove(buf, buf + plen, k);
142		*len = k;
143	}
144	return s;
145}
146