1  /*
  2   * Copyright (c) 2018 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   * Recompute public exponent, based on factor p and reduced private
 29   * exponent dp.
 30   */
 31  static uint32_t
 32  get_pubexp(const unsigned char *pbuf, size_t plen,
 33  	const unsigned char *dpbuf, size_t dplen)
 34  {
 35  	/*
 36  	 * dp is the inverse of e modulo p-1. If p = 3 mod 4, then
 37  	 * p-1 = 2*((p-1)/2). Taken modulo 2, e is odd and has inverse 1;
 38  	 * thus, dp must be odd.
 39  	 *
 40  	 * We compute the inverse of dp modulo (p-1)/2. This requires
 41  	 * first reducing dp modulo (p-1)/2 (this can be done with a
 42  	 * conditional subtract, no need to use the generic modular
 43  	 * reduction function); then, we use moddiv.
 44  	 */
 45  
 46  	uint32_t tmp[6 * ((BR_MAX_RSA_FACTOR + 61) / 31)];
 47  	uint32_t *p, *dp, *x;
 48  	size_t len;
 49  	uint32_t e;
 50  
 51  	/*
 52  	 * Compute actual factor length (in bytes) and check that it fits
 53  	 * under our size constraints.
 54  	 */
 55  	while (plen > 0 && *pbuf == 0) {
 56  		pbuf ++;
 57  		plen --;
 58  	}
 59  	if (plen == 0 || plen < 5 || plen > (BR_MAX_RSA_FACTOR / 8)) {
 60  		return 0;
 61  	}
 62  
 63  	/*
 64  	 * Compute actual reduced exponent length (in bytes) and check that
 65  	 * it is not longer than p.
 66  	 */
 67  	while (dplen > 0 && *dpbuf == 0) {
 68  		dpbuf ++;
 69  		dplen --;
 70  	}
 71  	if (dplen > plen || dplen == 0
 72  		|| (dplen == plen && dpbuf[0] > pbuf[0]))
 73  	{
 74  		return 0;
 75  	}
 76  
 77  	/*
 78  	 * Verify that p = 3 mod 4 and that dp is odd.
 79  	 */
 80  	if ((pbuf[plen - 1] & 3) != 3 || (dpbuf[dplen - 1] & 1) != 1) {
 81  		return 0;
 82  	}
 83  
 84  	/*
 85  	 * Decode p and compute (p-1)/2.
 86  	 */
 87  	p = tmp;
 88  	br_i31_decode(p, pbuf, plen);
 89  	len = (p[0] + 63) >> 5;
 90  	br_i31_rshift(p, 1);
 91  
 92  	/*
 93  	 * Decode dp and make sure its announced bit length matches that of
 94  	 * p (we already know that the size of dp, in bits, does not exceed
 95  	 * the size of p, so we just have to copy the header word).
 96  	 */
 97  	dp = p + len;
 98  	memset(dp, 0, len * sizeof *dp);
 99  	br_i31_decode(dp, dpbuf, dplen);
100  	dp[0] = p[0];
101  
102  	/*
103  	 * Subtract (p-1)/2 from dp if necessary.
104  	 */
105  	br_i31_sub(dp, p, NOT(br_i31_sub(dp, p, 0)));
106  
107  	/*
108  	 * If another subtraction is needed, then this means that the
109  	 * value was invalid. We don't care to leak information about
110  	 * invalid keys.
111  	 */
112  	if (br_i31_sub(dp, p, 0) == 0) {
113  		return 0;
114  	}
115  
116  	/*
117  	 * Invert dp modulo (p-1)/2. If the inversion fails, then the
118  	 * key value was invalid.
119  	 */
120  	x = dp + len;
121  	br_i31_zero(x, p[0]);
122  	x[1] = 1;
123  	if (br_i31_moddiv(x, dp, p, br_i31_ninv31(p[1]), x + len) == 0) {
124  		return 0;
125  	}
126  
127  	/*
128  	 * We now have an inverse. We must set it to zero (error) if its
129  	 * length is greater than 32 bits and/or if it is an even integer.
130  	 * Take care that the bit_length function returns an encoded
131  	 * bit length.
132  	 */
133  	e = (uint32_t)x[1] | ((uint32_t)x[2] << 31);
134  	e &= -LT(br_i31_bit_length(x + 1, len - 1), 34);
135  	e &= -(e & 1);
136  	return e;
137  }
138  
139  /* see bearssl_rsa.h */
140  uint32_t
141  br_rsa_i31_compute_pubexp(const br_rsa_private_key *sk)
142  {
143  	/*
144  	 * Get the public exponent from both p and q. This is the right
145  	 * exponent if we get twice the same value.
146  	 */
147  	uint32_t ep, eq;
148  
149  	ep = get_pubexp(sk->p, sk->plen, sk->dp, sk->dplen);
150  	eq = get_pubexp(sk->q, sk->qlen, sk->dq, sk->dqlen);
151  	return ep & -EQ(ep, eq);
152  }