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_i31_decode_reduce(uint32_t *x,
 30  	const void *src, size_t len, const uint32_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_i31_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 >> 5;
 61  	m_rbitlen = (m_ebitlen & 31) + (m_rbitlen << 5) - m_rbitlen;
 62  	mblen = (m_rbitlen + 7) >> 3;
 63  	k = mblen - 1;
 64  	if (k >= len) {
 65  		br_i31_decode(x, src, len);
 66  		x[0] = m_ebitlen;
 67  		return;
 68  	}
 69  	buf = src;
 70  	br_i31_decode(x, buf, k);
 71  	x[0] = m_ebitlen;
 72  
 73  	/*
 74  	 * Input remaining bytes, using 31-bit words.
 75  	 */
 76  	acc = 0;
 77  	acc_len = 0;
 78  	while (k < len) {
 79  		uint32_t v;
 80  
 81  		v = buf[k ++];
 82  		if (acc_len >= 23) {
 83  			acc_len -= 23;
 84  			acc <<= (8 - acc_len);
 85  			acc |= v >> acc_len;
 86  			br_i31_muladd_small(x, acc, m);
 87  			acc = v & (0xFF >> (8 - acc_len));
 88  		} else {
 89  			acc = (acc << 8) | v;
 90  			acc_len += 8;
 91  		}
 92  	}
 93  
 94  	/*
 95  	 * We may have some bits accumulated. We then perform a shift to
 96  	 * be able to inject these bits as a full 31-bit word.
 97  	 */
 98  	if (acc_len != 0) {
 99  		acc = (acc | (x[1] << acc_len)) & 0x7FFFFFFF;
100  		br_i31_rshift(x, 31 - acc_len);
101  		br_i31_muladd_small(x, acc, m);
102  	}
103  }