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

src/ec/ecdsa_i15_bits.c

@@ -0,0 +1,47 @@
+/*
+ * 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 inner.h */
+void
+br_ecdsa_i15_bits2int(uint16_t *x,
+	const void *src, size_t len, uint32_t ebitlen)
+{
+	uint32_t bitlen, hbitlen;
+	int sc;
+
+	bitlen = ebitlen - (ebitlen >> 4);
+	hbitlen = (uint32_t)len << 3;
+	if (hbitlen > bitlen) {
+		len = (bitlen + 7) >> 3;
+		sc = (int)((hbitlen - bitlen) & 7);
+	} else {
+		sc = 0;
+	}
+	br_i15_zero(x, ebitlen);
+	br_i15_decode(x, src, len);
+	br_i15_rshift(x, sc);
+	x[0] = ebitlen;
+}