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 * Recompute public exponent, based on factor p and reduced private
29 * exponent dp.
30 */
31static uint32_t
32get_pubexp(const unsigned char *pbuf, size_t plen,
33	const unsigned char *dpbuf, size_t dplen)
34{
35	/*
36	 * dp is the inverse of e modulo p-1. If p = 3 mod 4, then
37	 * p-1 = 2*((p-1)/2). Taken modulo 2, e is odd and has inverse 1;
38	 * thus, dp must be odd.
39	 *
40	 * We compute the inverse of dp modulo (p-1)/2. This requires
41	 * first reducing dp modulo (p-1)/2 (this can be done with a
42	 * conditional subtract, no need to use the generic modular
43	 * reduction function); then, we use moddiv.
44	 */
45
46	uint32_t tmp[6 * ((BR_MAX_RSA_FACTOR + 61) / 31)];
47	uint32_t *p, *dp, *x;
48	size_t len;
49	uint32_t e;
50
51	/*
52	 * Compute actual factor length (in bytes) and check that it fits
53	 * under our size constraints.
54	 */
55	while (plen > 0 && *pbuf == 0) {
56		pbuf ++;
57		plen --;
58	}
59	if (plen == 0 || plen < 5 || plen > (BR_MAX_RSA_FACTOR / 8)) {
60		return 0;
61	}
62
63	/*
64	 * Compute actual reduced exponent length (in bytes) and check that
65	 * it is not longer than p.
66	 */
67	while (dplen > 0 && *dpbuf == 0) {
68		dpbuf ++;
69		dplen --;
70	}
71	if (dplen > plen || dplen == 0
72		|| (dplen == plen && dpbuf[0] > pbuf[0]))
73	{
74		return 0;
75	}
76
77	/*
78	 * Verify that p = 3 mod 4 and that dp is odd.
79	 */
80	if ((pbuf[plen - 1] & 3) != 3 || (dpbuf[dplen - 1] & 1) != 1) {
81		return 0;
82	}
83
84	/*
85	 * Decode p and compute (p-1)/2.
86	 */
87	p = tmp;
88	br_i31_decode(p, pbuf, plen);
89	len = (p[0] + 63) >> 5;
90	br_i31_rshift(p, 1);
91
92	/*
93	 * Decode dp and make sure its announced bit length matches that of
94	 * p (we already know that the size of dp, in bits, does not exceed
95	 * the size of p, so we just have to copy the header word).
96	 */
97	dp = p + len;
98	memset(dp, 0, len * sizeof *dp);
99	br_i31_decode(dp, dpbuf, dplen);
100	dp[0] = p[0];
101
102	/*
103	 * Subtract (p-1)/2 from dp if necessary.
104	 */
105	br_i31_sub(dp, p, NOT(br_i31_sub(dp, p, 0)));
106
107	/*
108	 * If another subtraction is needed, then this means that the
109	 * value was invalid. We don't care to leak information about
110	 * invalid keys.
111	 */
112	if (br_i31_sub(dp, p, 0) == 0) {
113		return 0;
114	}
115
116	/*
117	 * Invert dp modulo (p-1)/2. If the inversion fails, then the
118	 * key value was invalid.
119	 */
120	x = dp + len;
121	br_i31_zero(x, p[0]);
122	x[1] = 1;
123	if (br_i31_moddiv(x, dp, p, br_i31_ninv31(p[1]), x + len) == 0) {
124		return 0;
125	}
126
127	/*
128	 * We now have an inverse. We must set it to zero (error) if its
129	 * length is greater than 32 bits and/or if it is an even integer.
130	 * Take care that the bit_length function returns an encoded
131	 * bit length.
132	 */
133	e = (uint32_t)x[1] | ((uint32_t)x[2] << 31);
134	e &= -LT(br_i31_bit_length(x + 1, len - 1), 34);
135	e &= -(e & 1);
136	return e;
137}
138
139/* see bearssl_rsa.h */
140uint32_t
141br_rsa_i31_compute_pubexp(const br_rsa_private_key *sk)
142{
143	/*
144	 * Get the public exponent from both p and q. This is the right
145	 * exponent if we get twice the same value.
146	 */
147	uint32_t ep, eq;
148
149	ep = get_pubexp(sk->p, sk->plen, sk->dp, sk->dplen);
150	eq = get_pubexp(sk->q, sk->qlen, sk->dq, sk->dqlen);
151	return ep & -EQ(ep, eq);
152}
153