1/*
2 * Copyright (c) 2016 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 * As a strict minimum, we need four buffers that can hold a
29 * modular integer.
30 */
31#define TLEN   (4 * (2 + ((BR_MAX_RSA_SIZE + 30) / 31)))
32
33/* see bearssl_rsa.h */
34uint32_t
35br_rsa_i31_public(unsigned char *x, size_t xlen,
36	const br_rsa_public_key *pk)
37{
38	const unsigned char *n;
39	size_t nlen;
40	uint32_t tmp[1 + TLEN];
41	uint32_t *m, *a, *t;
42	size_t fwlen;
43	long z;
44	uint32_t m0i, r;
45
46	/*
47	 * Get the actual length of the modulus, and see if it fits within
48	 * our stack buffer. We also check that the length of x[] is valid.
49	 */
50	n = pk->n;
51	nlen = pk->nlen;
52	while (nlen > 0 && *n == 0) {
53		n ++;
54		nlen --;
55	}
56	if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
57		return 0;
58	}
59	z = (long)nlen << 3;
60	fwlen = 1;
61	while (z > 0) {
62		z -= 31;
63		fwlen ++;
64	}
65	/*
66	 * Round up length to an even number.
67	 */
68	fwlen += (fwlen & 1);
69
70	/*
71	 * The modulus gets decoded into m[].
72	 * The value to exponentiate goes into a[].
73	 * The temporaries for modular exponentiation are in t[].
74	 */
75	m = tmp;
76	a = m + fwlen;
77	t = m + 2 * fwlen;
78
79	/*
80	 * Decode the modulus.
81	 */
82	br_i31_decode(m, n, nlen);
83	m0i = br_i31_ninv31(m[1]);
84
85	/*
86	 * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
87	 * an odd integer.
88	 */
89	r = m0i & 1;
90
91	/*
92	 * Decode x[] into a[]; we also check that its value is proper.
93	 */
94	r &= br_i31_decode_mod(a, x, xlen, m);
95
96	/*
97	 * Compute the modular exponentiation.
98	 */
99	br_i31_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen);
100
101	/*
102	 * Encode the result.
103	 */
104	br_i31_encode(x, xlen, a);
105	return r;
106}
107