New "i15" implementation of big integers (faster, and constant-time, on ARM Cortex M0/M0+); imported into EC, ECDSA, RSA.

src/rsa/rsa_i15_pub.c

@@ -0,0 +1,78 @@
+/*
+ * 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"
+
+/* see bearssl_rsa.h */
+uint32_t
+br_rsa_i15_public(unsigned char *x, size_t xlen,
+	const br_rsa_public_key *pk)
+{
+	const unsigned char *n;
+	size_t nlen;
+	uint16_t m[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
+	uint16_t a[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
+	uint16_t t1[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
+	uint16_t t2[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
+	uint16_t m0i;
+	uint32_t r;
+
+	/*
+	 * Get the actual length of the modulus, and see if it fits within
+	 * our stack buffer. We also check that the length of x[] is valid.
+	 */
+	n = pk->n;
+	nlen = pk->nlen;
+	while (nlen > 0 && *n == 0) {
+		n ++;
+		nlen --;
+	}
+	if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
+		return 0;
+	}
+	br_i15_decode(m, n, nlen);
+	m0i = br_i15_ninv15(m[1]);
+
+	/*
+	 * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
+	 * an odd integer.
+	 */
+	r = m0i & 1;
+
+	/*
+	 * Decode x[] into a[]; we also check that its value is proper.
+	 */
+	r &= br_i15_decode_mod(a, x, xlen, m);
+
+	/*
+	 * Compute the modular exponentiation.
+	 */
+	br_i15_modpow(a, pk->e, pk->elen, m, m0i, t1, t2);
+
+	/*
+	 * Encode the result.
+	 */
+	br_i15_encode(x, xlen, a);
+	return r;
+}