2 * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
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:
12 * The above copyright notice and this permission notice shall be
13 * included in all copies or substantial portions of the Software.
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
27 #define I15_LEN ((BR_MAX_EC_SIZE + 29) / 15)
28 #define POINT_LEN (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1))
30 /* see bearssl_ec.h */
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
)
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.
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
;
52 * If the curve is not supported, then report an error.
54 if (((impl
->supported_curves
>> pk
->curve
) & 1) == 0) {
59 * Get the curve parameters (generator and order).
76 * Signature length must be even.
84 * Public key point must have the proper size for this curve.
86 if (pk
->qlen
!= cd
->generator_len
) {
91 * Get modulus; then decode the r and s values. They must be
92 * lower than the modulus, and s must not be null.
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
)) {
100 if (!br_i15_decode_mod(s
, (const unsigned char *)sig
+ rlen
, rlen
, n
)) {
103 if (br_i15_iszero(s
)) {
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)).
116 br_i15_from_monty(s
, n
, n0i
);
117 memcpy(tx
, cd
->order
, nlen
);
119 br_i15_modpow(s
, tx
, nlen
, n
, n0i
, t1
, t2
);
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.
127 br_ecdsa_i15_bits2int(t1
, hash
, hash_len
, n
[0]);
128 br_i15_sub(t1
, n
, br_i15_sub(t1
, n
, 0) ^ 1);
131 * Multiply the (truncated, reduced) hash value with 1/s, result in
134 br_i15_montymul(t2
, t1
, s
, n
, n0i
);
135 br_i15_encode(ty
, nlen
, t2
);
138 * Multiply r with 1/s, result in t1, encoded in tx.
140 br_i15_montymul(t1
, r
, s
, n
, n0i
);
141 br_i15_encode(tx
, nlen
, t1
);
144 * Compute the point x*Q + y*G.
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
);
152 * Get the X coordinate, reduce modulo the curve order, and
153 * compare with the 'r' value.
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).
159 br_i15_zero(t1
, n
[0]);
160 br_i15_decode(t1
, &eU
[1], ulen
>> 1);
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
);