2 * Copyright (c) 2016 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 I31_LEN ((BR_MAX_EC_SIZE + 61) / 31)
28 #define POINT_LEN (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1))
29 #define ORDER_LEN ((BR_MAX_EC_SIZE + 7) >> 3)
31 /* see bearssl_ec.h */
33 br_ecdsa_i31_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
)
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
44 const br_ec_curve_def
*cd
;
45 uint32_t n
[I31_LEN
], r
[I31_LEN
], s
[I31_LEN
], x
[I31_LEN
];
46 uint32_t m
[I31_LEN
], k
[I31_LEN
], t1
[I31_LEN
], t2
[I31_LEN
];
47 unsigned char tt
[ORDER_LEN
<< 1];
48 unsigned char eU
[POINT_LEN
];
49 size_t hash_len
, nlen
, ulen
;
51 br_hmac_drbg_context drbg
;
54 * If the curve is not supported, then exit with an error.
56 if (((impl
->supported_curves
>> sk
->curve
) & 1) == 0) {
61 * Get the curve parameters (generator and order).
81 br_i31_decode(n
, cd
->order
, nlen
);
82 n0i
= br_i31_ninv31(n
[1]);
85 * Get private key as an i31 integer. This also checks that the
86 * private key is well-defined (not zero, and less than the
89 if (!br_i31_decode_mod(x
, sk
->x
, sk
->xlen
, n
)) {
92 if (br_i31_iszero(x
)) {
99 hash_len
= (hf
->desc
>> BR_HASHDESC_OUT_OFF
) & BR_HASHDESC_OUT_MASK
;
102 * Truncate and reduce the hash value modulo the curve order.
104 br_ecdsa_i31_bits2int(m
, hash_value
, hash_len
, n
[0]);
105 br_i31_sub(m
, n
, br_i31_sub(m
, n
, 0) ^ 1);
108 * RFC 6979 generation of the "k" value.
110 * The process uses HMAC_DRBG (with the hash function used to
111 * process the message that is to be signed). The seed is the
112 * concatenation of the encodings of the private key and
113 * the hash value (after truncation and modular reduction).
115 br_i31_encode(tt
, nlen
, x
);
116 br_i31_encode(tt
+ nlen
, nlen
, m
);
117 br_hmac_drbg_init(&drbg
, hf
, tt
, nlen
<< 1);
119 br_hmac_drbg_generate(&drbg
, tt
, nlen
);
120 br_ecdsa_i31_bits2int(k
, tt
, nlen
, n
[0]);
121 if (br_i31_iszero(k
)) {
124 if (br_i31_sub(k
, n
, 0)) {
130 * Compute k*G and extract the X coordinate, then reduce it
131 * modulo the curve order. Since we support only curves with
132 * prime order, that reduction is only a matter of computing
135 br_i31_encode(tt
, nlen
, k
);
136 ulen
= impl
->mulgen(eU
, tt
, nlen
, sk
->curve
);
137 br_i31_zero(r
, n
[0]);
138 br_i31_decode(r
, &eU
[1], ulen
>> 1);
140 br_i31_sub(r
, n
, br_i31_sub(r
, n
, 0) ^ 1);
143 * Compute 1/k in double-Montgomery representation. We do so by
144 * first converting _from_ Montgomery representation (twice),
145 * then using a modular exponentiation.
147 br_i31_from_monty(k
, n
, n0i
);
148 br_i31_from_monty(k
, n
, n0i
);
149 memcpy(tt
, cd
->order
, nlen
);
151 br_i31_modpow(k
, tt
, nlen
, n
, n0i
, t1
, t2
);
154 * Compute s = (m+xr)/k (mod n).
155 * The k[] array contains R^2/k (double-Montgomery representation);
156 * we thus can use direct Montgomery multiplications and conversions
157 * from Montgomery, avoiding any call to br_i31_to_monty() (which
160 br_i31_from_monty(m
, n
, n0i
);
161 br_i31_montymul(t1
, x
, r
, n
, n0i
);
162 ctl
= br_i31_add(t1
, m
, 1);
163 ctl
|= br_i31_sub(t1
, n
, 0) ^ 1;
164 br_i31_sub(t1
, n
, ctl
);
165 br_i31_montymul(s
, t1
, k
, n
, n0i
);
168 * Encode r and s in the signature.
170 br_i31_encode(sig
, nlen
, r
);
171 br_i31_encode((unsigned char *)sig
+ nlen
, nlen
, s
);