New "i15" implementation of big integers (faster, and constant-time, on ARM Cortex...

[BearSSL] / src / int / i15_ext1.c

diff --git a/src/int/i15_ext1.c b/src/int/i15_ext1.c
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+/*
+ * 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"
+
+/*
+ * This file contains some additional functions for "i15" big integers.
+ * These functions are needed to support ECDSA.
+ */
+
+/* see inner.h */
+void
+br_i15_rshift(uint16_t *x, int count)
+{
+       size_t u, len;
+       unsigned r;
+
+       len = (x[0] + 15) >> 4;
+       if (len == 0) {
+               return;
+       }
+       r = x[1] >> count;
+       for (u = 2; u <= len; u ++) {
+               unsigned w;
+
+               w = x[u];
+               x[u - 1] = ((w << (15 - count)) | r) & 0x7FFF;
+               r = w >> count;
+       }
+       x[len] = r;
+}
+
+/* see inner.h */
+uint32_t
+br_i15_bit_length(uint16_t *x, size_t xlen)
+{
+       uint32_t tw, twk;
+
+       tw = 0;
+       twk = 0;
+       while (xlen -- > 0) {
+               uint32_t w, c;
+
+               c = EQ(tw, 0);
+               w = x[xlen];
+               tw = MUX(c, w, tw);
+               twk = MUX(c, (uint32_t)xlen, twk);
+       }
+       return (twk << 4) + BIT_LENGTH(tw);
+}
+
+/* see inner.h */
+void
+br_i15_decode(uint16_t *x, const void *src, size_t len)
+{
+       const unsigned char *buf;
+       size_t v;
+       uint32_t acc;
+       int acc_len;
+
+       buf = src;
+       v = 1;
+       acc = 0;
+       acc_len = 0;
+       while (len -- > 0) {
+               uint32_t b;
+
+               b = buf[len];
+               acc |= (b << acc_len);
+               acc_len += 8;
+               if (acc_len >= 15) {
+                       x[v ++] = acc & 0x7FFF;
+                       acc_len -= 15;
+                       acc >>= 15;
+               }
+       }
+       if (acc_len != 0) {
+               x[v ++] = acc;
+       }
+       x[0] = br_i15_bit_length(x + 1, v - 1);
+}
+
+/* see inner.h */
+void
+br_i15_from_monty(uint16_t *x, const uint16_t *m, uint16_t m0i)
+{
+       size_t len, u, v;
+
+       len = (m[0] + 15) >> 4;
+       for (u = 0; u < len; u ++) {
+               uint32_t f, cc;
+
+               f = MUL15(x[1], m0i) & 0x7FFF;
+               cc = 0;
+               for (v = 0; v < len; v ++) {
+                       uint32_t z;
+
+                       z = (uint32_t)x[v + 1] + MUL15(f, m[v + 1]) + cc;
+                       cc = z >> 15;
+                       if (v != 0) {
+                               x[v] = z & 0x7FFF;
+                       }
+               }
+               x[len] = cc;
+       }
+
+       /*
+        * We may have to do an extra subtraction, but only if the
+        * value in x[] is indeed greater than or equal to that of m[],
+        * which is why we must do two calls (first call computes the
+        * carry, second call performs the subtraction only if the carry
+        * is 0).
+        */
+       br_i15_sub(x, m, NOT(br_i15_sub(x, m, 0)));
+}