_ Git - BearSSL/blob - src/int/i15_decred.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_decode_reduce(uint16_t *x,
  30         const void *src, size_t len, const uint16_t *m)
  31 {
  32         uint32_t m_ebitlen, m_rbitlen;
  33         size_t mblen, k;
  34         const unsigned char *buf;
  35         uint32_t acc;
  36         int acc_len;
  37
  38         /*
  39          * Get the encoded bit length.
  40          */
  41         m_ebitlen = m[0];
  42
  43         /*
  44          * Special case for an invalid (null) modulus.
  45          */
  46         if (m_ebitlen == 0) {
  47                 x[0] = 0;
  48                 return;
  49         }
  50
  51         /*
  52          * Clear the destination.
  53          */
  54         br_i15_zero(x, m_ebitlen);
  55
  56         /*
  57          * First decode directly as many bytes as possible. This requires
  58          * computing the actual bit length.
  59          */
  60         m_rbitlen = m_ebitlen >> 4;
  61         m_rbitlen = (m_ebitlen & 15) + (m_rbitlen << 4) - m_rbitlen;
  62         mblen = (m_rbitlen + 7) >> 3;
  63         k = mblen - 1;
  64         if (k >= len) {
  65                 br_i15_decode(x, src, len);
  66                 x[0] = m_ebitlen;
  67                 return;
  68         }
  69         buf = src;
  70         br_i15_decode(x, buf, k);
  71         x[0] = m_ebitlen;
  72
  73         /*
  74          * Input remaining bytes, using 15-bit words.
  75          */
  76         acc = 0;
  77         acc_len = 0;
  78         while (k < len) {
  79                 uint32_t v;
  80
  81                 v = buf[k ++];
  82                 acc = (acc << 8) | v;
  83                 acc_len += 8;
  84                 if (acc_len >= 15) {
  85                         br_i15_muladd_small(x, acc >> (acc_len - 15), m);
  86                         acc_len -= 15;
  87                         acc &= ~((uint32_t)-1 << acc_len);
  88                 }
  89         }
  90
  91         /*
  92          * We may have some bits accumulated. We then perform a shift to
  93          * be able to inject these bits as a full 15-bit word.
  94          */
  95         if (acc_len != 0) {
  96                 acc = (acc | (x[1] << acc_len)) & 0x7FFF;
  97                 br_i15_rshift(x, 15 - acc_len);
  98                 br_i15_muladd_small(x, acc, m);
  99         }
 100 }