1/*
2 * Copyright (c) 2017 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#define I15_LEN     ((BR_MAX_EC_SIZE + 29) / 15)
28#define POINT_LEN   (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1))
29#define ORDER_LEN   ((BR_MAX_EC_SIZE + 7) >> 3)
30
31/* see bearssl_ec.h */
32size_t
33br_ecdsa_i15_sign_raw(const br_ec_impl *impl,
34	const br_hash_class *hf, const void *hash_value,
35	const br_ec_private_key *sk, void *sig)
36{
37	/*
38	 * IMPORTANT: this code is fit only for curves with a prime
39	 * order. This is needed so that modular reduction of the X
40	 * coordinate of a point can be done with a simple subtraction.
41	 * We also rely on the last byte of the curve order to be distinct
42	 * from 0 and 1.
43	 */
44	const br_ec_curve_def *cd;
45	uint16_t n[I15_LEN], r[I15_LEN], s[I15_LEN], x[I15_LEN];
46	uint16_t m[I15_LEN], k[I15_LEN], t1[I15_LEN], t2[I15_LEN];
47	unsigned char tt[ORDER_LEN << 1];
48	unsigned char eU[POINT_LEN];
49	size_t hash_len, nlen, ulen;
50	uint16_t n0i;
51	uint32_t ctl;
52	br_hmac_drbg_context drbg;
53
54	/*
55	 * If the curve is not supported, then exit with an error.
56	 */
57	if (((impl->supported_curves >> sk->curve) & 1) == 0) {
58		return 0;
59	}
60
61	/*
62	 * Get the curve parameters (generator and order).
63	 */
64	switch (sk->curve) {
65	case BR_EC_secp256r1:
66		cd = &br_secp256r1;
67		break;
68	case BR_EC_secp384r1:
69		cd = &br_secp384r1;
70		break;
71	case BR_EC_secp521r1:
72		cd = &br_secp521r1;
73		break;
74	default:
75		return 0;
76	}
77
78	/*
79	 * Get modulus.
80	 */
81	nlen = cd->order_len;
82	br_i15_decode(n, cd->order, nlen);
83	n0i = br_i15_ninv15(n[1]);
84
85	/*
86	 * Get private key as an i15 integer. This also checks that the
87	 * private key is well-defined (not zero, and less than the
88	 * curve order).
89	 */
90	if (!br_i15_decode_mod(x, sk->x, sk->xlen, n)) {
91		return 0;
92	}
93	if (br_i15_iszero(x)) {
94		return 0;
95	}
96
97	/*
98	 * Get hash length.
99	 */
100	hash_len = (hf->desc >> BR_HASHDESC_OUT_OFF) & BR_HASHDESC_OUT_MASK;
101
102	/*
103	 * Truncate and reduce the hash value modulo the curve order.
104	 */
105	br_ecdsa_i15_bits2int(m, hash_value, hash_len, n[0]);
106	br_i15_sub(m, n, br_i15_sub(m, n, 0) ^ 1);
107
108	/*
109	 * RFC 6979 generation of the "k" value.
110	 *
111	 * The process uses HMAC_DRBG (with the hash function used to
112	 * process the message that is to be signed). The seed is the
113	 * concatenation of the encodings of the private key and
114	 * the hash value (after truncation and modular reduction).
115	 */
116	br_i15_encode(tt, nlen, x);
117	br_i15_encode(tt + nlen, nlen, m);
118	br_hmac_drbg_init(&drbg, hf, tt, nlen << 1);
119	for (;;) {
120		br_hmac_drbg_generate(&drbg, tt, nlen);
121		br_ecdsa_i15_bits2int(k, tt, nlen, n[0]);
122		if (br_i15_iszero(k)) {
123			continue;
124		}
125		if (br_i15_sub(k, n, 0)) {
126			break;
127		}
128	}
129
130	/*
131	 * Compute k*G and extract the X coordinate, then reduce it
132	 * modulo the curve order. Since we support only curves with
133	 * prime order, that reduction is only a matter of computing
134	 * a subtraction.
135	 */
136	br_i15_encode(tt, nlen, k);
137	ulen = impl->mulgen(eU, tt, nlen, sk->curve);
138	br_i15_zero(r, n[0]);
139	br_i15_decode(r, &eU[1], ulen >> 1);
140	r[0] = n[0];
141	br_i15_sub(r, n, br_i15_sub(r, n, 0) ^ 1);
142
143	/*
144	 * Compute 1/k in double-Montgomery representation. We do so by
145	 * first converting _from_ Montgomery representation (twice),
146	 * then using a modular exponentiation.
147	 */
148	br_i15_from_monty(k, n, n0i);
149	br_i15_from_monty(k, n, n0i);
150	memcpy(tt, cd->order, nlen);
151	tt[nlen - 1] -= 2;
152	br_i15_modpow(k, tt, nlen, n, n0i, t1, t2);
153
154	/*
155	 * Compute s = (m+xr)/k (mod n).
156	 * The k[] array contains R^2/k (double-Montgomery representation);
157	 * we thus can use direct Montgomery multiplications and conversions
158	 * from Montgomery, avoiding any call to br_i15_to_monty() (which
159	 * is slower).
160	 */
161	br_i15_from_monty(m, n, n0i);
162	br_i15_montymul(t1, x, r, n, n0i);
163	ctl = br_i15_add(t1, m, 1);
164	ctl |= br_i15_sub(t1, n, 0) ^ 1;
165	br_i15_sub(t1, n, ctl);
166	br_i15_montymul(s, t1, k, n, n0i);
167
168	/*
169	 * Encode r and s in the signature.
170	 */
171	br_i15_encode(sig, nlen, r);
172	br_i15_encode((unsigned char *)sig + nlen, nlen, s);
173	return nlen << 1;
174}
175