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/*
* Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include "inner.h"
#define I31_LEN ((BR_MAX_EC_SIZE + 61) / 31)
#define POINT_LEN (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1))
/* see bearssl_ec.h */
uint32_t
br_ecdsa_i31_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)
{
/*
* IMPORTANT: this code is fit only for curves with a prime
* order. This is needed so that modular reduction of the X
* coordinate of a point can be done with a simple subtraction.
*/
const br_ec_curve_def *cd;
uint32_t n[I31_LEN], r[I31_LEN], s[I31_LEN], t1[I31_LEN], t2[I31_LEN];
unsigned char tx[(BR_MAX_EC_SIZE + 7) >> 3];
unsigned char ty[(BR_MAX_EC_SIZE + 7) >> 3];
unsigned char eU[POINT_LEN];
size_t nlen, rlen, ulen;
uint32_t n0i, res;
/*
* If the curve is not supported, then report an error.
*/
if (((impl->supported_curves >> pk->curve) & 1) == 0) {
return 0;
}
/*
* Get the curve parameters (generator and order).
*/
switch (pk->curve) {
case BR_EC_secp256r1:
cd = &br_secp256r1;
break;
case BR_EC_secp384r1:
cd = &br_secp384r1;
break;
case BR_EC_secp521r1:
cd = &br_secp521r1;
break;
default:
return 0;
}
/*
* Signature length must be even.
*/
if (sig_len & 1) {
return 0;
}
rlen = sig_len >> 1;
/*
* Public key point must have the proper size for this curve.
*/
if (pk->qlen != cd->generator_len) {
return 0;
}
/*
* Get modulus; then decode the r and s values. They must be
* lower than the modulus, and s must not be null.
*/
nlen = cd->order_len;
br_i31_decode(n, cd->order, nlen);
n0i = br_i31_ninv31(n[1]);
if (!br_i31_decode_mod(r, sig, rlen, n)) {
return 0;
}
if (!br_i31_decode_mod(s, (const unsigned char *)sig + rlen, rlen, n)) {
return 0;
}
if (br_i31_iszero(s)) {
return 0;
}
/*
* Invert s. We do that with a modular exponentiation; we use
* the fact that for all the curves we support, the least
* significant byte is not 0 or 1, so we can subtract 2 without
* any carry to process.
* We also want 1/s in Montgomery representation, which can be
* done by converting _from_ Montgomery representation before
* the inversion (because (1/s)*R = 1/(s/R)).
*/
br_i31_from_monty(s, n, n0i);
memcpy(tx, cd->order, nlen);
tx[nlen - 1] -= 2;
br_i31_modpow(s, tx, nlen, n, n0i, t1, t2);
/*
* Truncate the hash to the modulus length (in bits) and reduce
* it modulo the curve order. The modular reduction can be done
* with a subtraction since the truncation already reduced the
* value to the modulus bit length.
*/
br_ecdsa_i31_bits2int(t1, hash, hash_len, n[0]);
br_i31_sub(t1, n, br_i31_sub(t1, n, 0) ^ 1);
/*
* Multiply the (truncated, reduced) hash value with 1/s, result in
* t2, encoded in ty.
*/
br_i31_montymul(t2, t1, s, n, n0i);
br_i31_encode(ty, nlen, t2);
/*
* Multiply r with 1/s, result in t1, encoded in tx.
*/
br_i31_montymul(t1, r, s, n, n0i);
br_i31_encode(tx, nlen, t1);
/*
* Compute the point x*Q + y*G.
*/
ulen = cd->generator_len;
memcpy(eU, pk->q, ulen);
res = impl->muladd(eU, NULL, ulen,
tx, nlen, ty, nlen, cd->curve);
/*
* Get the X coordinate, reduce modulo the curve order, and
* compare with the 'r' value.
*
* The modular reduction can be done with subtractions because
* we work with curves of prime order, so the curve order is
* close to the field order (Hasse's theorem).
*/
br_i31_zero(t1, n[0]);
br_i31_decode(t1, &eU[1], ulen >> 1);
t1[0] = n[0];
br_i31_sub(t1, n, br_i31_sub(t1, n, 0) ^ 1);
res &= ~br_i31_sub(t1, r, 1);
res &= br_i31_iszero(t1);
return res;
}
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