_ Git - BearSSL/blob - src/int/i15_reduce.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 void
  29 br_i15_reduce(uint16_t *x, const uint16_t *a, const uint16_t *m)
  30 {
  31         uint32_t m_bitlen, a_bitlen;
  32         size_t mlen, alen, u;
  33
  34         m_bitlen = m[0];
  35         mlen = (m_bitlen + 15) >> 4;
  36
  37         x[0] = m_bitlen;
  38         if (m_bitlen == 0) {
  39                 return;
  40         }
  41
  42         /*
  43          * If the source is shorter, then simply copy all words from a[]
  44          * and zero out the upper words.
  45          */
  46         a_bitlen = a[0];
  47         alen = (a_bitlen + 15) >> 4;
  48         if (a_bitlen < m_bitlen) {
  49                 memcpy(x + 1, a + 1, alen * sizeof *a);
  50                 for (u = alen; u < mlen; u ++) {
  51                         x[u + 1] = 0;
  52                 }
  53                 return;
  54         }
  55
  56         /*
  57          * The source length is at least equal to that of the modulus.
  58          * We must thus copy N-1 words, and input the remaining words
  59          * one by one.
  60          */
  61         memcpy(x + 1, a + 2 + (alen - mlen), (mlen - 1) * sizeof *a);
  62         x[mlen] = 0;
  63         for (u = 1 + alen - mlen; u > 0; u --) {
  64                 br_i15_muladd_small(x, a[u], m);
  65         }
  66 }