1*0957b409SSimon J. Gerraty /*
2*0957b409SSimon J. Gerraty * Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
3*0957b409SSimon J. Gerraty *
4*0957b409SSimon J. Gerraty * Permission is hereby granted, free of charge, to any person obtaining
5*0957b409SSimon J. Gerraty * a copy of this software and associated documentation files (the
6*0957b409SSimon J. Gerraty * "Software"), to deal in the Software without restriction, including
7*0957b409SSimon J. Gerraty * without limitation the rights to use, copy, modify, merge, publish,
8*0957b409SSimon J. Gerraty * distribute, sublicense, and/or sell copies of the Software, and to
9*0957b409SSimon J. Gerraty * permit persons to whom the Software is furnished to do so, subject to
10*0957b409SSimon J. Gerraty * the following conditions:
11*0957b409SSimon J. Gerraty *
12*0957b409SSimon J. Gerraty * The above copyright notice and this permission notice shall be
13*0957b409SSimon J. Gerraty * included in all copies or substantial portions of the Software.
14*0957b409SSimon J. Gerraty *
15*0957b409SSimon J. Gerraty * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16*0957b409SSimon J. Gerraty * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17*0957b409SSimon J. Gerraty * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18*0957b409SSimon J. Gerraty * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19*0957b409SSimon J. Gerraty * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20*0957b409SSimon J. Gerraty * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21*0957b409SSimon J. Gerraty * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
22*0957b409SSimon J. Gerraty * SOFTWARE.
23*0957b409SSimon J. Gerraty */
24*0957b409SSimon J. Gerraty
25*0957b409SSimon J. Gerraty #include "inner.h"
26*0957b409SSimon J. Gerraty
27*0957b409SSimon J. Gerraty #define I31_LEN ((BR_MAX_EC_SIZE + 61) / 31)
28*0957b409SSimon J. Gerraty #define POINT_LEN (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1))
29*0957b409SSimon J. Gerraty #define ORDER_LEN ((BR_MAX_EC_SIZE + 7) >> 3)
30*0957b409SSimon J. Gerraty
31*0957b409SSimon J. Gerraty /* see bearssl_ec.h */
32*0957b409SSimon J. Gerraty size_t
br_ecdsa_i31_sign_raw(const br_ec_impl * impl,const br_hash_class * hf,const void * hash_value,const br_ec_private_key * sk,void * sig)33*0957b409SSimon J. Gerraty br_ecdsa_i31_sign_raw(const br_ec_impl *impl,
34*0957b409SSimon J. Gerraty const br_hash_class *hf, const void *hash_value,
35*0957b409SSimon J. Gerraty const br_ec_private_key *sk, void *sig)
36*0957b409SSimon J. Gerraty {
37*0957b409SSimon J. Gerraty /*
38*0957b409SSimon J. Gerraty * IMPORTANT: this code is fit only for curves with a prime
39*0957b409SSimon J. Gerraty * order. This is needed so that modular reduction of the X
40*0957b409SSimon J. Gerraty * coordinate of a point can be done with a simple subtraction.
41*0957b409SSimon J. Gerraty * We also rely on the last byte of the curve order to be distinct
42*0957b409SSimon J. Gerraty * from 0 and 1.
43*0957b409SSimon J. Gerraty */
44*0957b409SSimon J. Gerraty const br_ec_curve_def *cd;
45*0957b409SSimon J. Gerraty uint32_t n[I31_LEN], r[I31_LEN], s[I31_LEN], x[I31_LEN];
46*0957b409SSimon J. Gerraty uint32_t m[I31_LEN], k[I31_LEN], t1[I31_LEN], t2[I31_LEN];
47*0957b409SSimon J. Gerraty unsigned char tt[ORDER_LEN << 1];
48*0957b409SSimon J. Gerraty unsigned char eU[POINT_LEN];
49*0957b409SSimon J. Gerraty size_t hash_len, nlen, ulen;
50*0957b409SSimon J. Gerraty uint32_t n0i, ctl;
51*0957b409SSimon J. Gerraty br_hmac_drbg_context drbg;
52*0957b409SSimon J. Gerraty
53*0957b409SSimon J. Gerraty /*
54*0957b409SSimon J. Gerraty * If the curve is not supported, then exit with an error.
55*0957b409SSimon J. Gerraty */
56*0957b409SSimon J. Gerraty if (((impl->supported_curves >> sk->curve) & 1) == 0) {
57*0957b409SSimon J. Gerraty return 0;
58*0957b409SSimon J. Gerraty }
59*0957b409SSimon J. Gerraty
60*0957b409SSimon J. Gerraty /*
61*0957b409SSimon J. Gerraty * Get the curve parameters (generator and order).
62*0957b409SSimon J. Gerraty */
63*0957b409SSimon J. Gerraty switch (sk->curve) {
64*0957b409SSimon J. Gerraty case BR_EC_secp256r1:
65*0957b409SSimon J. Gerraty cd = &br_secp256r1;
66*0957b409SSimon J. Gerraty break;
67*0957b409SSimon J. Gerraty case BR_EC_secp384r1:
68*0957b409SSimon J. Gerraty cd = &br_secp384r1;
69*0957b409SSimon J. Gerraty break;
70*0957b409SSimon J. Gerraty case BR_EC_secp521r1:
71*0957b409SSimon J. Gerraty cd = &br_secp521r1;
72*0957b409SSimon J. Gerraty break;
73*0957b409SSimon J. Gerraty default:
74*0957b409SSimon J. Gerraty return 0;
75*0957b409SSimon J. Gerraty }
76*0957b409SSimon J. Gerraty
77*0957b409SSimon J. Gerraty /*
78*0957b409SSimon J. Gerraty * Get modulus.
79*0957b409SSimon J. Gerraty */
80*0957b409SSimon J. Gerraty nlen = cd->order_len;
81*0957b409SSimon J. Gerraty br_i31_decode(n, cd->order, nlen);
82*0957b409SSimon J. Gerraty n0i = br_i31_ninv31(n[1]);
83*0957b409SSimon J. Gerraty
84*0957b409SSimon J. Gerraty /*
85*0957b409SSimon J. Gerraty * Get private key as an i31 integer. This also checks that the
86*0957b409SSimon J. Gerraty * private key is well-defined (not zero, and less than the
87*0957b409SSimon J. Gerraty * curve order).
88*0957b409SSimon J. Gerraty */
89*0957b409SSimon J. Gerraty if (!br_i31_decode_mod(x, sk->x, sk->xlen, n)) {
90*0957b409SSimon J. Gerraty return 0;
91*0957b409SSimon J. Gerraty }
92*0957b409SSimon J. Gerraty if (br_i31_iszero(x)) {
93*0957b409SSimon J. Gerraty return 0;
94*0957b409SSimon J. Gerraty }
95*0957b409SSimon J. Gerraty
96*0957b409SSimon J. Gerraty /*
97*0957b409SSimon J. Gerraty * Get hash length.
98*0957b409SSimon J. Gerraty */
99*0957b409SSimon J. Gerraty hash_len = (hf->desc >> BR_HASHDESC_OUT_OFF) & BR_HASHDESC_OUT_MASK;
100*0957b409SSimon J. Gerraty
101*0957b409SSimon J. Gerraty /*
102*0957b409SSimon J. Gerraty * Truncate and reduce the hash value modulo the curve order.
103*0957b409SSimon J. Gerraty */
104*0957b409SSimon J. Gerraty br_ecdsa_i31_bits2int(m, hash_value, hash_len, n[0]);
105*0957b409SSimon J. Gerraty br_i31_sub(m, n, br_i31_sub(m, n, 0) ^ 1);
106*0957b409SSimon J. Gerraty
107*0957b409SSimon J. Gerraty /*
108*0957b409SSimon J. Gerraty * RFC 6979 generation of the "k" value.
109*0957b409SSimon J. Gerraty *
110*0957b409SSimon J. Gerraty * The process uses HMAC_DRBG (with the hash function used to
111*0957b409SSimon J. Gerraty * process the message that is to be signed). The seed is the
112*0957b409SSimon J. Gerraty * concatenation of the encodings of the private key and
113*0957b409SSimon J. Gerraty * the hash value (after truncation and modular reduction).
114*0957b409SSimon J. Gerraty */
115*0957b409SSimon J. Gerraty br_i31_encode(tt, nlen, x);
116*0957b409SSimon J. Gerraty br_i31_encode(tt + nlen, nlen, m);
117*0957b409SSimon J. Gerraty br_hmac_drbg_init(&drbg, hf, tt, nlen << 1);
118*0957b409SSimon J. Gerraty for (;;) {
119*0957b409SSimon J. Gerraty br_hmac_drbg_generate(&drbg, tt, nlen);
120*0957b409SSimon J. Gerraty br_ecdsa_i31_bits2int(k, tt, nlen, n[0]);
121*0957b409SSimon J. Gerraty if (br_i31_iszero(k)) {
122*0957b409SSimon J. Gerraty continue;
123*0957b409SSimon J. Gerraty }
124*0957b409SSimon J. Gerraty if (br_i31_sub(k, n, 0)) {
125*0957b409SSimon J. Gerraty break;
126*0957b409SSimon J. Gerraty }
127*0957b409SSimon J. Gerraty }
128*0957b409SSimon J. Gerraty
129*0957b409SSimon J. Gerraty /*
130*0957b409SSimon J. Gerraty * Compute k*G and extract the X coordinate, then reduce it
131*0957b409SSimon J. Gerraty * modulo the curve order. Since we support only curves with
132*0957b409SSimon J. Gerraty * prime order, that reduction is only a matter of computing
133*0957b409SSimon J. Gerraty * a subtraction.
134*0957b409SSimon J. Gerraty */
135*0957b409SSimon J. Gerraty br_i31_encode(tt, nlen, k);
136*0957b409SSimon J. Gerraty ulen = impl->mulgen(eU, tt, nlen, sk->curve);
137*0957b409SSimon J. Gerraty br_i31_zero(r, n[0]);
138*0957b409SSimon J. Gerraty br_i31_decode(r, &eU[1], ulen >> 1);
139*0957b409SSimon J. Gerraty r[0] = n[0];
140*0957b409SSimon J. Gerraty br_i31_sub(r, n, br_i31_sub(r, n, 0) ^ 1);
141*0957b409SSimon J. Gerraty
142*0957b409SSimon J. Gerraty /*
143*0957b409SSimon J. Gerraty * Compute 1/k in double-Montgomery representation. We do so by
144*0957b409SSimon J. Gerraty * first converting _from_ Montgomery representation (twice),
145*0957b409SSimon J. Gerraty * then using a modular exponentiation.
146*0957b409SSimon J. Gerraty */
147*0957b409SSimon J. Gerraty br_i31_from_monty(k, n, n0i);
148*0957b409SSimon J. Gerraty br_i31_from_monty(k, n, n0i);
149*0957b409SSimon J. Gerraty memcpy(tt, cd->order, nlen);
150*0957b409SSimon J. Gerraty tt[nlen - 1] -= 2;
151*0957b409SSimon J. Gerraty br_i31_modpow(k, tt, nlen, n, n0i, t1, t2);
152*0957b409SSimon J. Gerraty
153*0957b409SSimon J. Gerraty /*
154*0957b409SSimon J. Gerraty * Compute s = (m+xr)/k (mod n).
155*0957b409SSimon J. Gerraty * The k[] array contains R^2/k (double-Montgomery representation);
156*0957b409SSimon J. Gerraty * we thus can use direct Montgomery multiplications and conversions
157*0957b409SSimon J. Gerraty * from Montgomery, avoiding any call to br_i31_to_monty() (which
158*0957b409SSimon J. Gerraty * is slower).
159*0957b409SSimon J. Gerraty */
160*0957b409SSimon J. Gerraty br_i31_from_monty(m, n, n0i);
161*0957b409SSimon J. Gerraty br_i31_montymul(t1, x, r, n, n0i);
162*0957b409SSimon J. Gerraty ctl = br_i31_add(t1, m, 1);
163*0957b409SSimon J. Gerraty ctl |= br_i31_sub(t1, n, 0) ^ 1;
164*0957b409SSimon J. Gerraty br_i31_sub(t1, n, ctl);
165*0957b409SSimon J. Gerraty br_i31_montymul(s, t1, k, n, n0i);
166*0957b409SSimon J. Gerraty
167*0957b409SSimon J. Gerraty /*
168*0957b409SSimon J. Gerraty * Encode r and s in the signature.
169*0957b409SSimon J. Gerraty */
170*0957b409SSimon J. Gerraty br_i31_encode(sig, nlen, r);
171*0957b409SSimon J. Gerraty br_i31_encode((unsigned char *)sig + nlen, nlen, s);
172*0957b409SSimon J. Gerraty return nlen << 1;
173*0957b409SSimon J. Gerraty }
174