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_ec.h */
28size_t
29br_ec_keygen(const br_prng_class **rng_ctx,
30	const br_ec_impl *impl, br_ec_private_key *sk,
31	void *kbuf, int curve)
32{
33	const unsigned char *order;
34	unsigned char *buf;
35	size_t len;
36	unsigned mask;
37
38	if (curve < 0 || curve >= 32
39		|| ((impl->supported_curves >> curve) & 1) == 0)
40	{
41		return 0;
42	}
43	order = impl->order(curve, &len);
44	while (len > 0 && *order == 0) {
45		order ++;
46		len --;
47	}
48	if (kbuf == NULL || len == 0) {
49		return len;
50	}
51	mask = order[0];
52	mask |= (mask >> 1);
53	mask |= (mask >> 2);
54	mask |= (mask >> 4);
55
56	/*
57	 * We generate sequences of random bits of the right size, until
58	 * the value is strictly lower than the curve order (we also
59	 * check for all-zero values, which are invalid).
60	 */
61	buf = kbuf;
62	for (;;) {
63		size_t u;
64		unsigned cc, zz;
65
66		(*rng_ctx)->generate(rng_ctx, buf, len);
67		buf[0] &= mask;
68		cc = 0;
69		u = len;
70		zz = 0;
71		while (u -- > 0) {
72			cc = ((unsigned)(buf[u] - order[u] - cc) >> 8) & 1;
73			zz |= buf[u];
74		}
75		if (cc != 0 && zz != 0) {
76			break;
77		}
78	}
79
80	if (sk != NULL) {
81		sk->curve = curve;
82		sk->x = buf;
83		sk->xlen = len;
84	}
85	return len;
86}
87