_ Git - BearSSL/blob - src/int/i31_modpow.c

   1 /*
   2  * Copyright (c) 2016 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 void
  29 br_i31_modpow(uint32_t *x,
  30         const unsigned char *e, size_t elen,
  31         const uint32_t *m, uint32_t m0i, uint32_t *t1, uint32_t *t2)
  32 {
  33         size_t mlen;
  34         uint32_t k;
  35
  36         /*
  37          * 'mlen' is the length of m[] expressed in bytes (including
  38          * the "bit length" first field).
  39          */
  40         mlen = ((m[0] + 63) >> 5) * sizeof m[0];
  41
  42         /*
  43          * Throughout the algorithm:
  44          * -- t1[] is in Montgomery representation; it contains x, x^2,
  45          * x^4, x^8...
  46          * -- The result is accumulated, in normal representation, in
  47          * the x[] array.
  48          * -- t2[] is used as destination buffer for each multiplication.
  49          *
  50          * Note that there is no need to call br_i32_from_monty().
  51          */
  52         memcpy(t1, x, mlen);
  53         br_i31_to_monty(t1, m);
  54         br_i31_zero(x, m[0]);
  55         x[1] = 1;
  56         for (k = 0; k < ((uint32_t)elen << 3); k ++) {
  57                 uint32_t ctl;
  58
  59                 ctl = (e[elen - 1 - (k >> 3)] >> (k & 7)) & 1;
  60                 br_i31_montymul(t2, x, t1, m, m0i);
  61                 CCOPY(ctl, x, t2, mlen);
  62                 br_i31_montymul(t2, t1, t1, m, m0i);
  63                 memcpy(t1, t2, mlen);
  64         }
  65 }