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 } 122