_ Git - BearSSL/blob - src/int/i15_modpow2.c

   1 /*
   2  * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining
   5  * a copy of this software and associated documentation files (the
   6  * "Software"), to deal in the Software without restriction, including
   7  * without limitation the rights to use, copy, modify, merge, publish,
   8  * distribute, sublicense, and/or sell copies of the Software, and to
   9  * permit persons to whom the Software is furnished to do so, subject to
  10  * the following conditions:
  11  *
  12  * The above copyright notice and this permission notice shall be
  13  * included in all copies or substantial portions of the Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  16  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  17  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  18  * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
  19  * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  20  * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  21  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22  * SOFTWARE.
  23  */
  24
  25 #include "inner.h"
  26
  27 /* see inner.h */
  28 uint32_t
  29 br_i15_modpow_opt(uint16_t *x,
  30         const unsigned char *e, size_t elen,
  31         const uint16_t *m, uint16_t m0i, uint16_t *tmp, size_t twlen)
  32 {
  33         size_t mlen, mwlen;
  34         uint16_t *t1, *t2, *base;
  35         size_t u, v;
  36         uint32_t acc;
  37         int acc_len, win_len;
  38
  39         /*
  40          * Get modulus size.
  41          */
  42         mwlen = (m[0] + 31) >> 4;
  43         mlen = mwlen * sizeof m[0];
  44         mwlen += (mwlen & 1);
  45         t1 = tmp;
  46         t2 = tmp + mwlen;
  47
  48         /*
  49          * Compute possible window size, with a maximum of 5 bits.
  50          * When the window has size 1 bit, we use a specific code
  51          * that requires only two temporaries. Otherwise, for a
  52          * window of k bits, we need 2^k+1 temporaries.
  53          */
  54         if (twlen < (mwlen << 1)) {
  55                 return 0;
  56         }
  57         for (win_len = 5; win_len > 1; win_len --) {
  58                 if ((((uint32_t)1 << win_len) + 1) * mwlen <= twlen) {
  59                         break;
  60                 }
  61         }
  62
  63         /*
  64          * Everything is done in Montgomery representation.
  65          */
  66         br_i15_to_monty(x, m);
  67
  68         /*
  69          * Compute window contents. If the window has size one bit only,
  70          * then t2 is set to x; otherwise, t2[0] is left untouched, and
  71          * t2[k] is set to x^k (for k >= 1).
  72          */
  73         if (win_len == 1) {
  74                 memcpy(t2, x, mlen);
  75         } else {
  76                 memcpy(t2 + mwlen, x, mlen);
  77                 base = t2 + mwlen;
  78                 for (u = 2; u < ((unsigned)1 << win_len); u ++) {
  79                         br_i15_montymul(base + mwlen, base, x, m, m0i);
  80                         base += mwlen;
  81                 }
  82         }
  83
  84         /*
  85          * We need to set x to 1, in Montgomery representation. This can
  86          * be done efficiently by setting the high word to 1, then doing
  87          * one word-sized shift.
  88          */
  89         br_i15_zero(x, m[0]);
  90         x[(m[0] + 15) >> 4] = 1;
  91         br_i15_muladd_small(x, 0, m);
  92
  93         /*
  94          * We process bits from most to least significant. At each
  95          * loop iteration, we have acc_len bits in acc.
  96          */
  97         acc = 0;
  98         acc_len = 0;
  99         while (acc_len > 0 || elen > 0) {
 100                 int i, k;
 101                 uint32_t bits;
 102
 103                 /*
 104                  * Get the next bits.
 105                  */
 106                 k = win_len;
 107                 if (acc_len < win_len) {
 108                         if (elen > 0) {
 109                                 acc = (acc << 8) | *e ++;
 110                                 elen --;
 111                                 acc_len += 8;
 112                         } else {
 113                                 k = acc_len;
 114                         }
 115                 }
 116                 bits = (acc >> (acc_len - k)) & (((uint32_t)1 << k) - 1);
 117                 acc_len -= k;
 118
 119                 /*
 120                  * We could get exactly k bits. Compute k squarings.
 121                  */
 122                 for (i = 0; i < k; i ++) {
 123                         br_i15_montymul(t1, x, x, m, m0i);
 124                         memcpy(x, t1, mlen);
 125                 }
 126
 127                 /*
 128                  * Window lookup: we want to set t2 to the window
 129                  * lookup value, assuming the bits are non-zero. If
 130                  * the window length is 1 bit only, then t2 is
 131                  * already set; otherwise, we do a constant-time lookup.
 132                  */
 133                 if (win_len > 1) {
 134                         br_i15_zero(t2, m[0]);
 135                         base = t2 + mwlen;
 136                         for (u = 1; u < ((uint32_t)1 << k); u ++) {
 137                                 uint32_t mask;
 138
 139                                 mask = -EQ(u, bits);
 140                                 for (v = 1; v < mwlen; v ++) {
 141                                         t2[v] |= mask & base[v];
 142                                 }
 143                                 base += mwlen;
 144                         }
 145                 }
 146
 147                 /*
 148                  * Multiply with the looked-up value. We keep the
 149                  * product only if the exponent bits are not all-zero.
 150                  */
 151                 br_i15_montymul(t1, x, t2, m, m0i);
 152                 CCOPY(NEQ(bits, 0), x, t1, mlen);
 153         }
 154
 155         /*
 156          * Convert back from Montgomery representation, and exit.
 157          */
 158         br_i15_from_monty(x, m, m0i);
 159         return 1;
 160 }