asn.1 ######### 结构 ========================================================== tag - length - value 其中，value中可以再嵌套 tag - length - value tag ========================================================== `Encoded Tag Bytes `_ `ASN.1 Listing of Universal Tags `_ 第8、7位标识tag class：{ universal 00, application 01, context-specific 10, private 11 } 第6位标识是否constructed: { 0 primitive, 1 constructed } 后5位标识tag number implicit vs explicit ---------------------------------------------------- `ASN.1 tagging principles `_ `IV. ASN.1 `_ `X.690 `_ 默认explicit .. code-block:: Type1 ::= VisibleString Type2 ::= [APPLICATION 3] IMPLICIT Type1 Type3 ::= [2] Type2 Type4 ::= [APPLICATION 7] IMPLICIT Type3 Type5 ::= [2] IMPLICIT Type2 value: Jones .. code-block:: Type1: 0x1A=0b00011010 0x05 0x4A6F6E657 Type2: Application 01, tag number 替换为3 0b01000011 = 0x43 0x05 0x4A6F6E657 Type3: 默认explicit, explicit为context-specific 10 且structured 1, tag number替换为2 0b10100010 = 0xa2 0x07 0x43 0x05 0x4A6F6E657 Type4: Application 01, tag number替换为07，structured 1不变 0b01100111 = 0x67 0x07 0x43 0x05 0x4A6F6E657 Type5: implicit为context-specific 10，tag number替换为2, 保持原来Type2的primitive不变 0b10000010 = 0x82 0x05 0x4A6F6E657 length ========================================================== `Encoded Length and Value Bytes `_ 第8位为0，标识short form length。后7位标识length值。 第8位为1，标识long form length。后7位标识length取值的字节数，随后的字节数取值即为实际length值。 第8位为1，如果后7位的字节数为0，则标识长度不定，遇0x00 0x00停止。 value ========================================================== oid ---------------------------------------------------- `OBJECT IDENTIFIER `_ OBJECT IDENTIFIER (oid) 的 value 进行了压缩 前两位合并成1个字节： x_1 * 40 + x_2 后续位数，如果<128，则映射为单字节；如果>=128，则第8bit 置1，后7bits做为128的倍数处理，<128的余数单独编列为1字节。 bit string ---------------------------------------------------- `BIT STRING `_ bit string 的 value 部分的首个字节，标识了将该bit string长度填充为8的倍数所需的bit数 示例 ========================================================== `Reading encode asn file manually `_ 30 82 02 10 04 01 56 … ---------------------------------------------------------- .. code-block:: tag: 0x30 = 0b00110000 class = 00 universal constructed = 1 yes tag number = 0b10000 = 16 = SEQUENCE and SEQUENCE OF length: 0x82 = 0b10000010, long form length : 1 length's bytes number = 2 length = 0x0210 = 528 value: 04 01 56 ... 30 80 04 03 56 78 90 00 00 ---------------------------------------------------- tag: 同前 length: 0x80 = 0b10000000, long form length : 1 标识后面字节数不定 value: 04 03 56 78 90 : tag 04, length 03, value 56 78 90 00 00 : tag 00, length 00 df 82 02 05 12 34 56 78 90 ---------------------------------------------------- tag: df 82 02 0xdf = 1101 1111 : 11 class private, 0 primitive, 11111 全1标识long tag encoding 0x82 = 1000 0010 : 1 后面的字节还是tag number取值， 0b0000010 = 2 0x02 = 0000 0010 : 0 是tag number取值的最后一个字节，0b0000010 = 2 tag number = 0b00000100000010 = 258 length: 05 value: 12 34 56 78 90 oid 1.3.6.1.4.1.311.21.20 ---------------------------------------------------- .. code-block:: 06 09 ; OBJECT_ID (9 Bytes) | 2b 06 01 04 01 82 37 15 14 0x06 : tag object 0x09 : length 0x2b : 1*40 + 3 = 0x2b 311 = 128*2 + 55 = 0b10000010 + 0b00110111 = 0x82 + 0x37 bit string 011011100101110111 ---------------------------------------------------- 0110 1110 0101 1101 11xx xxxx 应填充6个bit: 0110 1110 0101 1101 1100 0000 tag: 0x03 length: 0x04 value: 0x06 填充6个bit，后面3个字节以填充后的padding直接转换 0x6e 0x5d 0xc0 => 03 04 06 6e 5d c0 (short form length) => 03 81 04 06 6e 5d c0 (long form of length octets) => 23 09 03 03 00 6e 5d 03 02 06 c0 (constructed) DER vs BER ========================================================== DER是BER的子集, 对每个ASN.1值只有唯一一种编码方法 DER对短型长度、长型长度、隐式标签简单定长、显式标签结构化定长等场景做了限制。 参考资料 ========================================================== - `ASN.1: Introduction `_ - `ASN.1/BER/DER `_ - `Parsing BER and DER encoded ASN.1 Objects `_ - `Introduction to ASN.1 Syntax and Encoding `_ - `ASN.1 Complete `_ - `ASN.1 JavaScript decoder `_ - `A Layman's Guide to a Subset of ASN.1, BER, and DER `_