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  /*
 28   * As a strict minimum, we need four buffers that can hold a
 29   * modular integer.
 30   */
 31  #define TLEN   (4 * (2 + ((BR_MAX_RSA_SIZE + 30) / 31)))
 32  
 33  /* see bearssl_rsa.h */
 34  uint32_t
 35  br_rsa_i31_public(unsigned char *x, size_t xlen,
 36  	const br_rsa_public_key *pk)
 37  {
 38  	const unsigned char *n;
 39  	size_t nlen;
 40  	uint32_t tmp[1 + TLEN];
 41  	uint32_t *m, *a, *t;
 42  	size_t fwlen;
 43  	long z;
 44  	uint32_t m0i, r;
 45  
 46  	/*
 47  	 * Get the actual length of the modulus, and see if it fits within
 48  	 * our stack buffer. We also check that the length of x[] is valid.
 49  	 */
 50  	n = pk->n;
 51  	nlen = pk->nlen;
 52  	while (nlen > 0 && *n == 0) {
 53  		n ++;
 54  		nlen --;
 55  	}
 56  	if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
 57  		return 0;
 58  	}
 59  	z = (long)nlen << 3;
 60  	fwlen = 1;
 61  	while (z > 0) {
 62  		z -= 31;
 63  		fwlen ++;
 64  	}
 65  	/*
 66  	 * Round up length to an even number.
 67  	 */
 68  	fwlen += (fwlen & 1);
 69  
 70  	/*
 71  	 * The modulus gets decoded into m[].
 72  	 * The value to exponentiate goes into a[].
 73  	 * The temporaries for modular exponentiation are in t[].
 74  	 */
 75  	m = tmp;
 76  	a = m + fwlen;
 77  	t = m + 2 * fwlen;
 78  
 79  	/*
 80  	 * Decode the modulus.
 81  	 */
 82  	br_i31_decode(m, n, nlen);
 83  	m0i = br_i31_ninv31(m[1]);
 84  
 85  	/*
 86  	 * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
 87  	 * an odd integer.
 88  	 */
 89  	r = m0i & 1;
 90  
 91  	/*
 92  	 * Decode x[] into a[]; we also check that its value is proper.
 93  	 */
 94  	r &= br_i31_decode_mod(a, x, xlen, m);
 95  
 96  	/*
 97  	 * Compute the modular exponentiation.
 98  	 */
 99  	br_i31_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen);
100  
101  	/*
102  	 * Encode the result.
103  	 */
104  	br_i31_encode(x, xlen, a);
105  	return r;
106  }