--- /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))
+#define ORDER_LEN ((BR_MAX_EC_SIZE + 7) >> 3)
+
+/* see bearssl_ec.h */
+size_t
+br_ecdsa_i15_sign_raw(const br_ec_impl *impl,
+ const br_hash_class *hf, const void *hash_value,
+ const br_ec_private_key *sk, void *sig)
+{
+ /*
+ * 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.
+ * We also rely on the last byte of the curve order to be distinct
+ * from 0 and 1.
+ */
+ const br_ec_curve_def *cd;
+ uint16_t n[I15_LEN], r[I15_LEN], s[I15_LEN], x[I15_LEN];
+ uint16_t m[I15_LEN], k[I15_LEN], t1[I15_LEN], t2[I15_LEN];
+ unsigned char tt[ORDER_LEN << 1];
+ unsigned char eU[POINT_LEN];
+ size_t hash_len, nlen, ulen;
+ uint16_t n0i;
+ uint32_t ctl;
+ br_hmac_drbg_context drbg;
+
+ /*
+ * If the curve is not supported, then exit with an error.
+ */
+ if (((impl->supported_curves >> sk->curve) & 1) == 0) {
+ return 0;
+ }
+
+ /*
+ * Get the curve parameters (generator and order).
+ */
+ switch (sk->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;
+ }
+
+ /*
+ * Get modulus.
+ */
+ nlen = cd->order_len;
+ br_i15_decode(n, cd->order, nlen);
+ n0i = br_i15_ninv15(n[1]);
+
+ /*
+ * Get private key as an i15 integer. This also checks that the
+ * private key is well-defined (not zero, and less than the
+ * curve order).
+ */
+ if (!br_i15_decode_mod(x, sk->x, sk->xlen, n)) {
+ return 0;
+ }
+ if (br_i15_iszero(x)) {
+ return 0;
+ }
+
+ /*
+ * Get hash length.
+ */
+ hash_len = (hf->desc >> BR_HASHDESC_OUT_OFF) & BR_HASHDESC_OUT_MASK;
+
+ /*
+ * Truncate and reduce the hash value modulo the curve order.
+ */
+ br_ecdsa_i15_bits2int(m, hash_value, hash_len, n[0]);
+ br_i15_sub(m, n, br_i15_sub(m, n, 0) ^ 1);
+
+ /*
+ * RFC 6979 generation of the "k" value.
+ *
+ * The process uses HMAC_DRBG (with the hash function used to
+ * process the message that is to be signed). The seed is the
+ * concatenation of the encodings of the private key and
+ * the hash value (after truncation and modular reduction).
+ */
+ br_i15_encode(tt, nlen, x);
+ br_i15_encode(tt + nlen, nlen, m);
+ br_hmac_drbg_init(&drbg, hf, tt, nlen << 1);
+ for (;;) {
+ br_hmac_drbg_generate(&drbg, tt, nlen);
+ br_ecdsa_i15_bits2int(k, tt, nlen, n[0]);
+ if (br_i15_iszero(k)) {
+ continue;
+ }
+ if (br_i15_sub(k, n, 0)) {
+ break;
+ }
+ }
+
+ /*
+ * Compute k*G and extract the X coordinate, then reduce it
+ * modulo the curve order. Since we support only curves with
+ * prime order, that reduction is only a matter of computing
+ * a subtraction.
+ */
+ ulen = cd->generator_len;
+ memcpy(eU, cd->generator, ulen);
+ br_i15_encode(tt, nlen, k);
+ if (!impl->mul(eU, ulen, tt, nlen, sk->curve)) {
+ /*
+ * Point multiplication may fail here only if the
+ * EC implementation does not support the curve, or the
+ * private key is incorrect (x is a multiple of the curve
+ * order).
+ */
+ return 0;
+ }
+ br_i15_zero(r, n[0]);
+ br_i15_decode(r, &eU[1], ulen >> 1);
+ r[0] = n[0];
+ br_i15_sub(r, n, br_i15_sub(r, n, 0) ^ 1);
+
+ /*
+ * Compute 1/k in double-Montgomery representation. We do so by
+ * first converting _from_ Montgomery representation (twice),
+ * then using a modular exponentiation.
+ */
+ br_i15_from_monty(k, n, n0i);
+ br_i15_from_monty(k, n, n0i);
+ memcpy(tt, cd->order, nlen);
+ tt[nlen - 1] -= 2;
+ br_i15_modpow(k, tt, nlen, n, n0i, t1, t2);
+
+ /*
+ * Compute s = (m+xr)/k (mod n).
+ * The k[] array contains R^2/k (double-Montgomery representation);
+ * we thus can use direct Montgomery multiplications and conversions
+ * from Montgomery, avoiding any call to br_i15_to_monty() (which
+ * is slower).
+ */
+ br_i15_from_monty(m, n, n0i);
+ br_i15_montymul(t1, x, r, n, n0i);
+ ctl = br_i15_add(t1, m, 1);
+ ctl |= br_i15_sub(t1, n, 0) ^ 1;
+ br_i15_sub(t1, n, ctl);
+ br_i15_montymul(s, t1, k, n, n0i);
+
+ /*
+ * Encode r and s in the signature.
+ */
+ br_i15_encode(sig, nlen, r);
+ br_i15_encode((unsigned char *)sig + nlen, nlen, s);
+ return nlen << 1;
+}
projects / BearSSL / blobdiff