1 /* $OpenBSD: asn1_lib.c,v 1.51 2021/12/25 07:04:03 jsing Exp $ */ 2 /* 3 * Copyright (c) 2021 Joel Sing <jsing@openbsd.org> 4 * 5 * Permission to use, copy, modify, and distribute this software for any 6 * purpose with or without fee is hereby granted, provided that the above 7 * copyright notice and this permission notice appear in all copies. 8 * 9 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 10 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 11 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 12 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 13 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 14 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 15 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 16 */ 17 18 #include <limits.h> 19 20 #include "bytestring.h" 21 22 static int 23 asn1_get_identifier_cbs(CBS *cbs, int der_mode, uint8_t *out_class, 24 int *out_constructed, uint32_t *out_tag_number) 25 { 26 uint8_t tag_class, tag_val; 27 int tag_constructed; 28 uint32_t tag_number; 29 30 /* 31 * Decode ASN.1 identifier octets - see ITU-T X.690 section 8.1.2. 32 */ 33 34 *out_class = 0; 35 *out_constructed = 0; 36 *out_tag_number = 0; 37 38 if (!CBS_get_u8(cbs, &tag_val)) 39 return 0; 40 41 /* 42 * ASN.1 tag class, encoding (primitive or constructed) and tag number 43 * are encoded in one or more identifier octets - the first octet 44 * contains the 2 bit tag class, the 1 bit encoding type and 5 bits 45 * of tag number. 46 * 47 * For tag numbers larger than 30 (0x1e) the 5 bit tag number in the 48 * first octet is set to all ones (0x1f) - the tag number is then 49 * encoded in subsequent octets - each of which have a one bit 50 * continuation flag and 7 bits of tag number in big-endian form. 51 * The encoding should not contain leading zeros but can for BER. 52 */ 53 tag_class = (tag_val >> 6) & 0x3; 54 tag_constructed = (tag_val >> 5) & 0x1; 55 tag_number = tag_val & 0x1f; 56 57 /* Long form. */ 58 if (tag_number == 0x1f) { 59 tag_number = 0; 60 do { 61 if (!CBS_get_u8(cbs, &tag_val)) 62 return 0; 63 if (der_mode && tag_number == 0 && tag_val == 0x80) 64 return 0; 65 if (tag_number > (UINT32_MAX >> 7)) 66 return 0; 67 tag_number = tag_number << 7 | (tag_val & 0x7f); 68 } while ((tag_val & 0x80) != 0); 69 } 70 71 *out_class = tag_class; 72 *out_constructed = tag_constructed; 73 *out_tag_number = tag_number; 74 75 return 1; 76 } 77 78 static int 79 asn1_get_length_cbs(CBS *cbs, int der_mode, int *out_indefinite, 80 uint32_t *out_length) 81 { 82 uint8_t len_bytes; 83 uint32_t length; 84 uint8_t val; 85 86 /* 87 * Decode ASN.1 length octets - see ITU-T X.690 section 8.1.3. 88 */ 89 90 *out_length = 0; 91 *out_indefinite = 0; 92 93 if (!CBS_get_u8(cbs, &val)) 94 return 0; 95 96 /* 97 * Short form - length is encoded in the lower 7 bits of a single byte. 98 */ 99 if (val < 0x80) { 100 *out_length = val; 101 return 1; 102 } 103 104 /* 105 * Indefinite length - content continues until an End of Content (EOC) 106 * marker is reached. Must be used with constructed encoding. 107 */ 108 if (val == 0x80) { 109 *out_indefinite = 1; 110 return 1; 111 } 112 113 /* 114 * Long form - the lower 7 bits of the first byte specifies the number 115 * of bytes used to encode the length, the following bytes specify the 116 * length in big-endian form. The encoding should not contain leading 117 * zeros but can for BER. A length value of 0x7f is invalid. 118 */ 119 if ((len_bytes = val & 0x7f) == 0x7f) 120 return 0; 121 122 length = 0; 123 124 while (len_bytes-- > 0) { 125 if (!CBS_get_u8(cbs, &val)) 126 return 0; 127 if (der_mode && length == 0 && val == 0) 128 return 0; 129 if (length > (UINT32_MAX >> 8)) 130 return 0; 131 length = (length << 8) | val; 132 } 133 134 *out_length = length; 135 136 return 1; 137 } 138 139 int 140 asn1_get_object_cbs(CBS *cbs, int der_mode, uint8_t *out_tag_class, 141 int *out_constructed, uint32_t *out_tag_number, int *out_indefinite, 142 uint32_t *out_length) 143 { 144 int constructed, indefinite; 145 uint32_t tag_number, length; 146 uint8_t tag_class; 147 148 *out_tag_class = 0; 149 *out_constructed = 0; 150 *out_tag_number = 0; 151 *out_indefinite = 0; 152 *out_length = 0; 153 154 if (!asn1_get_identifier_cbs(cbs, der_mode, &tag_class, &constructed, 155 &tag_number)) 156 return 0; 157 if (!asn1_get_length_cbs(cbs, der_mode, &indefinite, &length)) 158 return 0; 159 160 /* Indefinite length can only be used with constructed encoding. */ 161 if (indefinite && !constructed) 162 return 0; 163 164 *out_tag_class = tag_class; 165 *out_constructed = constructed; 166 *out_tag_number = tag_number; 167 *out_indefinite = indefinite; 168 *out_length = length; 169 170 return 1; 171 } 172