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/* see inner.h */
28uint32_t
29br_rsa_pss_sig_pad(const br_prng_class **rng,
30	const br_hash_class *hf_data, const br_hash_class *hf_mgf1,
31	const unsigned char *hash, size_t salt_len,
32	uint32_t n_bitlen, unsigned char *x)
33{
34	size_t xlen, hash_len;
35	br_hash_compat_context hc;
36	unsigned char *salt, *seed;
37
38	hash_len = br_digest_size(hf_data);
39
40	/*
41	 * The padded string is one bit smaller than the modulus;
42	 * notably, if the modulus length is equal to 1 modulo 8, then
43	 * the padded string will be one _byte_ smaller, and the first
44	 * byte will be set to 0. We apply these transformations here.
45	 */
46	n_bitlen --;
47	if ((n_bitlen & 7) == 0) {
48		*x ++ = 0;
49	}
50	xlen = (n_bitlen + 7) >> 3;
51
52	/*
53	 * Check that the modulus is large enough for the hash value
54	 * length combined with the intended salt length.
55	 */
56	if (hash_len > xlen || salt_len > xlen
57		|| (hash_len + salt_len + 2) > xlen)
58	{
59		return 0;
60	}
61
62	/*
63	 * Produce a random salt.
64	 */
65	salt = x + xlen - hash_len - salt_len - 1;
66	if (salt_len != 0) {
67		(*rng)->generate(rng, salt, salt_len);
68	}
69
70	/*
71	 * Compute the seed for MGF1.
72	 */
73	seed = x + xlen - hash_len - 1;
74	hf_data->init(&hc.vtable);
75	memset(seed, 0, 8);
76	hf_data->update(&hc.vtable, seed, 8);
77	hf_data->update(&hc.vtable, hash, hash_len);
78	hf_data->update(&hc.vtable, salt, salt_len);
79	hf_data->out(&hc.vtable, seed);
80
81	/*
82	 * Prepare string PS (padded salt). The salt is already at the
83	 * right place.
84	 */
85	memset(x, 0, xlen - salt_len - hash_len - 2);
86	x[xlen - salt_len - hash_len - 2] = 0x01;
87
88	/*
89	 * Generate the mask and XOR it into PS.
90	 */
91	br_mgf1_xor(x, xlen - hash_len - 1, hf_mgf1, seed, hash_len);
92
93	/*
94	 * Clear the top bits to ensure the value is lower than the
95	 * modulus.
96	 */
97	x[0] &= 0xFF >> (((uint32_t)xlen << 3) - n_bitlen);
98
99	/*
100	 * The seed (H) is already in the right place. We just set the
101	 * last byte.
102	 */
103	x[xlen - 1] = 0xBC;
104
105	return 1;
106}
107