+/*
+ * 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 */
+uint32_t
+br_i15_modpow_opt(uint16_t *x,
+ const unsigned char *e, size_t elen,
+ const uint16_t *m, uint16_t m0i, uint16_t *tmp, size_t twlen)
+{
+ size_t mlen, mwlen;
+ uint16_t *t1, *t2, *base;
+ size_t u, v;
+ uint32_t acc;
+ int acc_len, win_len;
+
+ /*
+ * Get modulus size.
+ */
+ mwlen = (m[0] + 31) >> 4;
+ mlen = mwlen * sizeof m[0];
+ t1 = tmp;
+ t2 = tmp + mwlen;
+
+ /*
+ * Compute possible window size, with a maximum of 5 bits.
+ * When the window has size 1 bit, we use a specific code
+ * that requires only two temporaries. Otherwise, for a
+ * window of k bits, we need 2^k+1 temporaries.
+ */
+ if (twlen < (mwlen << 1)) {
+ return 0;
+ }
+ for (win_len = 5; win_len > 1; win_len --) {
+ if ((((uint32_t)1 << win_len) + 1) * mwlen <= twlen) {
+ break;
+ }
+ }
+
+ /*
+ * Everything is done in Montgomery representation.
+ */
+ br_i15_to_monty(x, m);
+
+ /*
+ * Compute window contents. If the window has size one bit only,
+ * then t2 is set to x; otherwise, t2[0] is left untouched, and
+ * t2[k] is set to x^k (for k >= 1).
+ */
+ if (win_len == 1) {
+ memcpy(t2, x, mlen);
+ } else {
+ memcpy(t2 + mwlen, x, mlen);
+ base = t2 + mwlen;
+ for (u = 2; u < ((unsigned)1 << win_len); u ++) {
+ br_i15_montymul(base + mwlen, base, x, m, m0i);
+ base += mwlen;
+ }
+ }
+
+ /*
+ * We need to set x to 1, in Montgomery representation. This can
+ * be done efficiently by setting the high word to 1, then doing
+ * one word-sized shift.
+ */
+ br_i15_zero(x, m[0]);
+ x[mwlen - 1] = 1;
+ br_i15_muladd_small(x, 0, m);
+
+ /*
+ * We process bits from most to least significant. At each
+ * loop iteration, we have acc_len bits in acc.
+ */
+ acc = 0;
+ acc_len = 0;
+ while (acc_len > 0 || elen > 0) {
+ int i, k;
+ uint32_t bits;
+
+ /*
+ * Get the next bits.
+ */
+ k = win_len;
+ if (acc_len < win_len) {
+ if (elen > 0) {
+ acc = (acc << 8) | *e ++;
+ elen --;
+ acc_len += 8;
+ } else {
+ k = acc_len;
+ }
+ }
+ bits = (acc >> (acc_len - k)) & (((uint32_t)1 << k) - 1);
+ acc_len -= k;
+
+ /*
+ * We could get exactly k bits. Compute k squarings.
+ */
+ for (i = 0; i < k; i ++) {
+ br_i15_montymul(t1, x, x, m, m0i);
+ memcpy(x, t1, mlen);
+ }
+
+ /*
+ * Window lookup: we want to set t2 to the window
+ * lookup value, assuming the bits are non-zero. If
+ * the window length is 1 bit only, then t2 is
+ * already set; otherwise, we do a constant-time lookup.
+ */
+ if (win_len > 1) {
+ br_i15_zero(t2, m[0]);
+ base = t2 + mwlen;
+ for (u = 1; u < ((uint32_t)1 << k); u ++) {
+ uint32_t m;
+
+ m = -EQ(u, bits);
+ for (v = 1; v < mwlen; v ++) {
+ t2[v] |= m & base[v];
+ }
+ base += mwlen;
+ }
+ }
+
+ /*
+ * Multiply with the looked-up value. We keep the
+ * product only if the exponent bits are not all-zero.
+ */
+ br_i15_montymul(t1, x, t2, m, m0i);
+ CCOPY(NEQ(bits, 0), x, t1, mlen);
+ }
+
+ /*
+ * Convert back from Montgomery representation, and exit.
+ */
+ br_i15_from_monty(x, m, m0i);
+ return 1;
+}
projects / BearSSL / commitdiff