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  /* see bearssl_rsa.h */
 28  uint32_t
 29  br_rsa_pkcs1_sig_unpad(const unsigned char *sig, size_t sig_len,
 30  	const unsigned char *hash_oid, size_t hash_len,
 31  	unsigned char *hash_out)
 32  {
 33  	static const unsigned char pad1[] = {
 34  		0x00, 0x01, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
 35  	};
 36  
 37  	unsigned char pad2[43];
 38  	size_t u, x2, x3, pad_len, zlen;
 39  
 40  	if (sig_len < 11) {
 41  		return 0;
 42  	}
 43  
 44  	/*
 45  	 * Expected format:
 46  	 *  00 01 FF ... FF 00 30 x1 30 x2 06 x3 OID [ 05 00 ] 04 x4 HASH
 47  	 *
 48  	 * with the following rules:
 49  	 *
 50  	 *  -- Total length is that of the modulus and the signature
 51  	 *     (this was already verified by br_rsa_i31_public()).
 52  	 *
 53  	 *  -- There are at least eight bytes of value 0xFF.
 54  	 *
 55  	 *  -- x4 is equal to the hash length (hash_len).
 56  	 *
 57  	 *  -- x3 is equal to the encoded OID value length (so x3 is the
 58  	 *     first byte of hash_oid[]).
 59  	 *
 60  	 *  -- If the "05 00" is present, then x2 == x3 + 4; otherwise,
 61  	 *     x2 == x3 + 2.
 62  	 *
 63  	 *  -- x1 == x2 + x4 + 4.
 64  	 *
 65  	 * So the total length after the last "FF" is either x3 + x4 + 11
 66  	 * (with the "05 00") or x3 + x4 + 9 (without the "05 00").
 67  	 */
 68  
 69  	/*
 70  	 * Check the "00 01 FF .. FF 00" with at least eight 0xFF bytes.
 71  	 * The comparison is valid because we made sure that the signature
 72  	 * is at least 11 bytes long.
 73  	 */
 74  	if (memcmp(sig, pad1, sizeof pad1) != 0) {
 75  		return 0;
 76  	}
 77  	for (u = sizeof pad1; u < sig_len; u ++) {
 78  		if (sig[u] != 0xFF) {
 79  			break;
 80  		}
 81  	}
 82  
 83  	/*
 84  	 * Remaining length is sig_len - u bytes (including the 00 just
 85  	 * after the last FF). This must be equal to one of the two
 86  	 * possible values (depending on whether the "05 00" sequence is
 87  	 * present or not).
 88  	 */
 89  	if (hash_oid == NULL) {
 90  		if (sig_len - u != hash_len + 1 || sig[u] != 0x00) {
 91  			return 0;
 92  		}
 93  	} else {
 94  		x3 = hash_oid[0];
 95  		pad_len = x3 + 9;
 96  		memset(pad2, 0, pad_len);
 97  		zlen = sig_len - u - hash_len;
 98  		if (zlen == pad_len) {
 99  			x2 = x3 + 2;
100  		} else if (zlen == pad_len + 2) {
101  			x2 = x3 + 4;
102  			pad_len = zlen;
103  			pad2[pad_len - 4] = 0x05;
104  		} else {
105  			return 0;
106  		}
107  		pad2[1] = 0x30;
108  		pad2[2] = x2 + hash_len + 4;
109  		pad2[3] = 0x30;
110  		pad2[4] = x2;
111  		pad2[5] = 0x06;
112  		memcpy(pad2 + 6, hash_oid, x3 + 1);
113  		pad2[pad_len - 2] = 0x04;
114  		pad2[pad_len - 1] = hash_len;
115  		if (memcmp(pad2, sig + u, pad_len) != 0) {
116  			return 0;
117  		}
118  	}
119  	memcpy(hash_out, sig + sig_len - hash_len, hash_len);
120  	return 1;
121  }