--- /dev/null
+/*
+ * Copyright (c) 2017 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 I15_LEN ((BR_MAX_EC_SIZE + 29) / 15)
+#define POINT_LEN (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1))
+
+/* see bearssl_ec.h */
+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)
+{
+ /*
+ * 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;
+ uint16_t n[I15_LEN], r[I15_LEN], s[I15_LEN], t1[I15_LEN], t2[I15_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;
+ uint16_t n0i;
+ uint32_t 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_i15_decode(n, cd->order, nlen);
+ n0i = br_i15_ninv15(n[1]);
+ if (!br_i15_decode_mod(r, sig, rlen, n)) {
+ return 0;
+ }
+ if (!br_i15_decode_mod(s, (const unsigned char *)sig + rlen, rlen, n)) {
+ return 0;
+ }
+ if (br_i15_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_i15_from_monty(s, n, n0i);
+ memcpy(tx, cd->order, nlen);
+ tx[nlen - 1] -= 2;
+ br_i15_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_i15_bits2int(t1, hash, hash_len, n[0]);
+ br_i15_sub(t1, n, br_i15_sub(t1, n, 0) ^ 1);
+
+ /*
+ * Multiply the (truncated, reduced) hash value with 1/s, result in
+ * t2, encoded in ty.
+ */
+ br_i15_montymul(t2, t1, s, n, n0i);
+ br_i15_encode(ty, nlen, t2);
+
+ /*
+ * Multiply r with 1/s, result in t1, encoded in tx.
+ */
+ br_i15_montymul(t1, r, s, n, n0i);
+ br_i15_encode(tx, nlen, t1);
+
+ /*
+ * Compute the point x*Q + y*G.
+ */
+ ulen = cd->generator_len;
+ memcpy(eU, pk->q, ulen);
+ res = impl->muladd(eU, cd->generator, 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_i15_zero(t1, n[0]);
+ br_i15_decode(t1, &eU[1], ulen >> 1);
+ t1[0] = n[0];
+ br_i15_sub(t1, n, br_i15_sub(t1, n, 0) ^ 1);
+ res &= ~br_i15_sub(t1, r, 1);
+ res &= br_i15_iszero(t1);
+ return res;
+}
projects / BearSSL / blobdiff