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

[BearSSL] / src / int / i15_ext2.c

diff --git a/src/int/i15_ext2.c b/src/int/i15_ext2.c
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+++ b/src/int/i15_ext2.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 RSA.
+ */
+
+/* see inner.h */
+void
+br_i15_decode_reduce(uint16_t *x,
+       const void *src, size_t len, const uint16_t *m)
+{
+       uint32_t m_ebitlen, m_rbitlen;
+       size_t mblen, k;
+       const unsigned char *buf;
+       uint32_t acc;
+       int acc_len;
+
+       /*
+        * Get the encoded bit length.
+        */
+       m_ebitlen = m[0];
+
+       /*
+        * Special case for an invalid (null) modulus.
+        */
+       if (m_ebitlen == 0) {
+               x[0] = 0;
+               return;
+       }
+
+       /*
+        * Clear the destination.
+        */
+       br_i15_zero(x, m_ebitlen);
+
+       /*
+        * First decode directly as many bytes as possible. This requires
+        * computing the actual bit length.
+        */
+       m_rbitlen = m_ebitlen >> 4;
+       m_rbitlen = (m_ebitlen & 15) + (m_rbitlen << 4) - m_rbitlen;
+       mblen = (m_rbitlen + 7) >> 3;
+       k = mblen - 1;
+       if (k >= len) {
+               br_i15_decode(x, src, len);
+               x[0] = m_ebitlen;
+               return;
+       }
+       buf = src;
+       br_i15_decode(x, buf, k);
+       x[0] = m_ebitlen;
+
+       /*
+        * Input remaining bytes, using 15-bit words.
+        */
+       acc = 0;
+       acc_len = 0;
+       while (k < len) {
+               uint32_t v;
+
+               v = buf[k ++];
+               acc = (acc << 8) | v;
+               acc_len += 8;
+               if (acc_len >= 15) {
+                       br_i15_muladd_small(x, acc >> (acc_len - 15), m);
+                       acc_len -= 15;
+                       acc &= ~((uint32_t)-1 << acc_len);
+               }
+       }
+
+       /*
+        * We may have some bits accumulated. We then perform a shift to
+        * be able to inject these bits as a full 15-bit word.
+        */
+       if (acc_len != 0) {
+               acc = (acc | (x[1] << acc_len)) & 0x7FFF;
+               br_i15_rshift(x, 15 - acc_len);
+               br_i15_muladd_small(x, acc, m);
+       }
+}
+
+/* see inner.h */
+void
+br_i15_reduce(uint16_t *x, const uint16_t *a, const uint16_t *m)
+{
+       uint32_t m_bitlen, a_bitlen;
+       size_t mlen, alen, u;
+
+       m_bitlen = m[0];
+       mlen = (m_bitlen + 15) >> 4;
+
+       x[0] = m_bitlen;
+       if (m_bitlen == 0) {
+               return;
+       }
+
+       /*
+        * If the source is shorter, then simply copy all words from a[]
+        * and zero out the upper words.
+        */
+       a_bitlen = a[0];
+       alen = (a_bitlen + 15) >> 4;
+       if (a_bitlen < m_bitlen) {
+               memcpy(x + 1, a + 1, alen * sizeof *a);
+               for (u = alen; u < mlen; u ++) {
+                       x[u + 1] = 0;
+               }
+               return;
+       }
+
+       /*
+        * The source length is at least equal to that of the modulus.
+        * We must thus copy N-1 words, and input the remaining words
+        * one by one.
+        */
+       memcpy(x + 1, a + 2 + (alen - mlen), (mlen - 1) * sizeof *a);
+       x[mlen] = 0;
+       for (u = 1 + alen - mlen; u > 0; u --) {
+               br_i15_muladd_small(x, a[u], m);
+       }
+}
+
+/* see inner.h */
+void
+br_i15_mulacc(uint16_t *d, const uint16_t *a, const uint16_t *b)
+{
+       size_t alen, blen, u;
+
+       alen = (a[0] + 15) >> 4;
+       blen = (b[0] + 15) >> 4;
+       d[0] = a[0] + b[0];
+       for (u = 0; u < blen; u ++) {
+               uint32_t f;
+               size_t v;
+               uint32_t cc;
+
+               f = b[1 + u];
+               cc = 0;
+               for (v = 0; v < alen; v ++) {
+                       uint32_t z;
+
+                       z = (uint32_t)d[1 + u + v] + MUL15(f, a[1 + v]) + cc;
+                       cc = z >> 15;
+                       d[1 + u + v] = z & 0x7FFF;
+               }
+               d[1 + u + alen] = cc;
+       }
+}