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
30 /* see bearssl_ec.h */
31 uint32_t
br_ecdsa_i15_vrfy_raw(const br_ec_impl * impl,const void * hash,size_t hash_len,const br_ec_public_key * pk,const void * sig,size_t sig_len)32 br_ecdsa_i15_vrfy_raw(const br_ec_impl *impl,
33 const void *hash, size_t hash_len,
34 const br_ec_public_key *pk,
35 const void *sig, size_t sig_len)
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 */
42 const br_ec_curve_def *cd;
43 uint16_t n[I15_LEN], r[I15_LEN], s[I15_LEN], t1[I15_LEN], t2[I15_LEN];
44 unsigned char tx[(BR_MAX_EC_SIZE + 7) >> 3];
45 unsigned char ty[(BR_MAX_EC_SIZE + 7) >> 3];
46 unsigned char eU[POINT_LEN];
47 size_t nlen, rlen, ulen;
48 uint16_t n0i;
49 uint32_t res;
50
51 /*
52 * If the curve is not supported, then report an error.
53 */
54 if (((impl->supported_curves >> pk->curve) & 1) == 0) {
55 return 0;
56 }
57
58 /*
59 * Get the curve parameters (generator and order).
60 */
61 switch (pk->curve) {
62 case BR_EC_secp256r1:
63 cd = &br_secp256r1;
64 break;
65 case BR_EC_secp384r1:
66 cd = &br_secp384r1;
67 break;
68 case BR_EC_secp521r1:
69 cd = &br_secp521r1;
70 break;
71 default:
72 return 0;
73 }
74
75 /*
76 * Signature length must be even.
77 */
78 if (sig_len & 1) {
79 return 0;
80 }
81 rlen = sig_len >> 1;
82
83 /*
84 * Public key point must have the proper size for this curve.
85 */
86 if (pk->qlen != cd->generator_len) {
87 return 0;
88 }
89
90 /*
91 * Get modulus; then decode the r and s values. They must be
92 * lower than the modulus, and s must not be null.
93 */
94 nlen = cd->order_len;
95 br_i15_decode(n, cd->order, nlen);
96 n0i = br_i15_ninv15(n[1]);
97 if (!br_i15_decode_mod(r, sig, rlen, n)) {
98 return 0;
99 }
100 if (!br_i15_decode_mod(s, (const unsigned char *)sig + rlen, rlen, n)) {
101 return 0;
102 }
103 if (br_i15_iszero(s)) {
104 return 0;
105 }
106
107 /*
108 * Invert s. We do that with a modular exponentiation; we use
109 * the fact that for all the curves we support, the least
110 * significant byte is not 0 or 1, so we can subtract 2 without
111 * any carry to process.
112 * We also want 1/s in Montgomery representation, which can be
113 * done by converting _from_ Montgomery representation before
114 * the inversion (because (1/s)*R = 1/(s/R)).
115 */
116 br_i15_from_monty(s, n, n0i);
117 memcpy(tx, cd->order, nlen);
118 tx[nlen - 1] -= 2;
119 br_i15_modpow(s, tx, nlen, n, n0i, t1, t2);
120
121 /*
122 * Truncate the hash to the modulus length (in bits) and reduce
123 * it modulo the curve order. The modular reduction can be done
124 * with a subtraction since the truncation already reduced the
125 * value to the modulus bit length.
126 */
127 br_ecdsa_i15_bits2int(t1, hash, hash_len, n[0]);
128 br_i15_sub(t1, n, br_i15_sub(t1, n, 0) ^ 1);
129
130 /*
131 * Multiply the (truncated, reduced) hash value with 1/s, result in
132 * t2, encoded in ty.
133 */
134 br_i15_montymul(t2, t1, s, n, n0i);
135 br_i15_encode(ty, nlen, t2);
136
137 /*
138 * Multiply r with 1/s, result in t1, encoded in tx.
139 */
140 br_i15_montymul(t1, r, s, n, n0i);
141 br_i15_encode(tx, nlen, t1);
142
143 /*
144 * Compute the point x*Q + y*G.
145 */
146 ulen = cd->generator_len;
147 memcpy(eU, pk->q, ulen);
148 res = impl->muladd(eU, NULL, ulen,
149 tx, nlen, ty, nlen, cd->curve);
150
151 /*
152 * Get the X coordinate, reduce modulo the curve order, and
153 * compare with the 'r' value.
154 *
155 * The modular reduction can be done with subtractions because
156 * we work with curves of prime order, so the curve order is
157 * close to the field order (Hasse's theorem).
158 */
159 br_i15_zero(t1, n[0]);
160 br_i15_decode(t1, &eU[1], ulen >> 1);
161 t1[0] = n[0];
162 br_i15_sub(t1, n, br_i15_sub(t1, n, 0) ^ 1);
163 res &= ~br_i15_sub(t1, r, 1);
164 res &= br_i15_iszero(t1);
165 return res;
166 }
167