_ Git - BearSSL/blob - src/int/i32_decmod.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 uint32_t
  29 br_i32_decode_mod(uint32_t *x, const void *src, size_t len, const uint32_t *m)
  30 {
  31         const unsigned char *buf;
  32         uint32_t r;
  33         size_t u, v, mlen;
  34
  35         buf = src;
  36
  37         /*
  38          * First pass: determine whether the value fits. The 'r' value
  39          * will contain the comparison result, as 0x00000000 (value is
  40          * equal to the modulus), 0x00000001 (value is greater than the
  41          * modulus), or 0xFFFFFFFF (value is lower than the modulus).
  42          */
  43         mlen = (m[0] + 7) >> 3;
  44         r = 0;
  45         for (u = (mlen > len) ? mlen : len; u > 0; u --) {
  46                 uint32_t mb, xb;
  47
  48                 v = u - 1;
  49                 if (v >= mlen) {
  50                         mb = 0;
  51                 } else {
  52                         mb = (m[1 + (v >> 2)] >> ((v & 3) << 3)) & 0xFF;
  53                 }
  54                 if (v >= len) {
  55                         xb = 0;
  56                 } else {
  57                         xb = buf[len - u];
  58                 }
  59                 r = MUX(EQ(r, 0), (uint32_t)CMP(xb, mb), r);
  60         }
  61
  62         /*
  63          * Only r == 0xFFFFFFFF is acceptable. We want to set r to 0xFF if
  64          * the value fits, 0x00 otherwise.
  65          */
  66         r >>= 24;
  67         br_i32_zero(x, m[0]);
  68         u = (mlen > len) ? len : mlen;
  69         while (u > 0) {
  70                 uint32_t xb;
  71
  72                 xb = buf[len - u] & r;
  73                 u --;
  74                 x[1 + (u >> 2)] |= xb << ((u & 3) << 3);
  75         }
  76         return r >> 7;
  77 }