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  }