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 bearssl_rsa.h */
28size_t
29br_rsa_i15_compute_modulus(void *n, const br_rsa_private_key *sk)
30{
31	uint16_t tmp[4 * (((BR_MAX_RSA_SIZE / 2) + 14) / 15) + 5];
32	uint16_t *t, *p, *q;
33	const unsigned char *pbuf, *qbuf;
34	size_t nlen, plen, qlen, tlen;
35
36	/*
37	 * Compute actual byte and lengths for p and q.
38	 */
39	pbuf = sk->p;
40	plen = sk->plen;
41	while (plen > 0 && *pbuf == 0) {
42		pbuf ++;
43		plen --;
44	}
45	qbuf = sk->q;
46	qlen = sk->qlen;
47	while (qlen > 0 && *qbuf == 0) {
48		qbuf ++;
49		qlen --;
50	}
51
52	t = tmp;
53	tlen = (sizeof tmp) / (sizeof tmp[0]);
54
55	/*
56	 * Decode p.
57	 */
58	if ((15 * tlen) < (plen << 3) + 15) {
59		return 0;
60	}
61	br_i15_decode(t, pbuf, plen);
62	p = t;
63	plen = (p[0] + 31) >> 4;
64	t += plen;
65	tlen -= plen;
66
67	/*
68	 * Decode q.
69	 */
70	if ((15 * tlen) < (qlen << 3) + 15) {
71		return 0;
72	}
73	br_i15_decode(t, qbuf, qlen);
74	q = t;
75	qlen = (q[0] + 31) >> 4;
76	t += qlen;
77	tlen -= qlen;
78
79	/*
80	 * Computation can proceed only if we have enough room for the
81	 * modulus.
82	 */
83	if (tlen < (plen + qlen + 1)) {
84		return 0;
85	}
86
87	/*
88	 * Private key already contains the modulus bit length, from which
89	 * we can infer the output length. Even if n is NULL, we still had
90	 * to decode p and q to make sure that the product can be computed.
91	 */
92	nlen = (sk->n_bitlen + 7) >> 3;
93	if (n != NULL) {
94		br_i15_zero(t, p[0]);
95		br_i15_mulacc(t, p, q);
96		br_i15_encode(n, nlen, t);
97	}
98	return nlen;
99}
100