_ Git - BearSSL/blob - src/int/i32_decred.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_i32_decode_reduce(uint32_t *x,
  30         const void *src, size_t len, const uint32_t *m)
  31 {
  32         uint32_t m_bitlen;
  33         size_t mblen, k, q;
  34         const unsigned char *buf;
  35
  36         m_bitlen = m[0];
  37
  38         /*
  39          * Special case for an invalid modulus.
  40          */
  41         if (m_bitlen == 0) {
  42                 x[0] = 0;
  43                 return;
  44         }
  45
  46         /*
  47          * Clear the destination.
  48          */
  49         br_i32_zero(x, m_bitlen);
  50
  51         /*
  52          * First decode directly as many bytes as possible without
  53          * reduction, taking care to leave a number of bytes which
  54          * is a multiple of 4.
  55          */
  56         mblen = (m_bitlen + 7) >> 3;
  57         k = mblen - 1;
  58
  59         /*
  60          * Up to k bytes can be safely decoded.
  61          */
  62         if (k >= len) {
  63                 br_i32_decode(x, src, len);
  64                 x[0] = m_bitlen;
  65                 return;
  66         }
  67
  68         /*
  69          * We want to first inject some bytes with direct decoding,
  70          * then extra bytes by whole 32-bit words. First compute
  71          * the size that should be injected that way.
  72          */
  73         buf = src;
  74         q = (len - k + 3) & ~(size_t)3;
  75
  76         /*
  77          * It may happen that this is more than what we already have
  78          * (by at most 3 bytes). Such a case may happen only with
  79          * a very short modulus. In that case, we must process the first
  80          * bytes "manually".
  81          */
  82         if (q > len) {
  83                 int i;
  84                 uint32_t w;
  85
  86                 w = 0;
  87                 for (i = 0; i < 4; i ++) {
  88                         w <<= 8;
  89                         if (q <= len) {
  90                                 w |= buf[len - q];
  91                         }
  92                         q --;
  93                 }
  94                 br_i32_muladd_small(x, w, m);
  95         } else {
  96                 br_i32_decode(x, buf, len - q);
  97                 x[0] = m_bitlen;
  98         }
  99
 100         /*
 101          * At that point, we have exactly q bytes to inject, and q is
 102          * a multiple of 4.
 103          */
 104         for (k = len - q; k < len; k += 4) {
 105                 br_i32_muladd_small(x, br_dec32be(buf + k), m);
 106         }
 107 }