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 */
28  size_t
29  br_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  }