1  /*
  2   * Copyright (c) 2017 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 + 14) / 15)))
 32  
 33  /* see bearssl_rsa.h */
 34  uint32_t
 35  br_rsa_i15_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  	uint16_t tmp[1 + TLEN];
 41  	uint16_t *m, *a, *t;
 42  	size_t fwlen;
 43  	long z;
 44  	uint16_t m0i;
 45  	uint32_t r;
 46  
 47  	/*
 48  	 * Get the actual length of the modulus, and see if it fits within
 49  	 * our stack buffer. We also check that the length of x[] is valid.
 50  	 */
 51  	n = pk->n;
 52  	nlen = pk->nlen;
 53  	while (nlen > 0 && *n == 0) {
 54  		n ++;
 55  		nlen --;
 56  	}
 57  	if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
 58  		return 0;
 59  	}
 60  	z = (long)nlen << 3;
 61  	fwlen = 1;
 62  	while (z > 0) {
 63  		z -= 15;
 64  		fwlen ++;
 65  	}
 66  	/*
 67  	 * Round up length to an even number.
 68  	 */
 69  	fwlen += (fwlen & 1);
 70  
 71  	/*
 72  	 * The modulus gets decoded into m[].
 73  	 * The value to exponentiate goes into a[].
 74  	 * The temporaries for modular exponentiations are in t[].
 75  	 *
 76  	 * We want the first value word of each integer to be aligned
 77  	 * on a 32-bit boundary.
 78  	 */
 79  	m = tmp;
 80  	if (((uintptr_t)m & 2) == 0) {
 81  		m ++;
 82  	}
 83  	a = m + fwlen;
 84  	t = m + 2 * fwlen;
 85  
 86  	/*
 87  	 * Decode the modulus.
 88  	 */
 89  	br_i15_decode(m, n, nlen);
 90  	m0i = br_i15_ninv15(m[1]);
 91  
 92  	/*
 93  	 * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
 94  	 * an odd integer.
 95  	 */
 96  	r = m0i & 1;
 97  
 98  	/*
 99  	 * Decode x[] into a[]; we also check that its value is proper.
100  	 */
101  	r &= br_i15_decode_mod(a, x, xlen, m);
102  
103  	/*
104  	 * Compute the modular exponentiation.
105  	 */
106  	br_i15_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen);
107  
108  	/*
109  	 * Encode the result.
110  	 */
111  	br_i15_encode(x, xlen, a);
112  	return r;
113  }