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_modpow(uint32_t *x,
30  	const unsigned char *e, size_t elen,
31  	const uint32_t *m, uint32_t m0i, uint32_t *t1, uint32_t *t2)
32  {
33  	size_t mlen;
34  	uint32_t k;
35  
36  	/*
37  	 * 'mlen' is the length of m[] expressed in bytes (including
38  	 * the "bit length" first field).
39  	 */
40  	mlen = ((m[0] + 63) >> 5) * sizeof m[0];
41  
42  	/*
43  	 * Throughout the algorithm:
44  	 * -- t1[] is in Montgomery representation; it contains x, x^2,
45  	 * x^4, x^8...
46  	 * -- The result is accumulated, in normal representation, in
47  	 * the x[] array.
48  	 * -- t2[] is used as destination buffer for each multiplication.
49  	 *
50  	 * Note that there is no need to call br_i32_from_monty().
51  	 */
52  	memcpy(t1, x, mlen);
53  	br_i31_to_monty(t1, m);
54  	br_i31_zero(x, m[0]);
55  	x[1] = 1;
56  	for (k = 0; k < ((uint32_t)elen << 3); k ++) {
57  		uint32_t ctl;
58  
59  		ctl = (e[elen - 1 - (k >> 3)] >> (k & 7)) & 1;
60  		br_i31_montymul(t2, x, t1, m, m0i);
61  		CCOPY(ctl, x, t2, mlen);
62  		br_i31_montymul(t2, t1, t1, m, m0i);
63  		memcpy(t1, t2, mlen);
64  	}
65  }