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 */
28 uint32_t
29 br_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