ASN.1
2019-06-06 [read as txt or PDF]
Abstract Syntax Notation One (ASN.1) allows defining data structures that can be serialized and deserialized in a cross-platform way.
Basic Encoding Rules (BER) specify a self-describing and self-delimiting format for encoding ASN.1 data structures.
Distinguished Encoding Rules (DER) is a restricted variant of BER thas is unequivocal.
DER is a tag-length-value (TLV) encoding:
- tag (two forms):
- low-tag-number form: One octet. Bits 8 and 7 specify the class, bit 6 has value 0 indicating that the encoding is primitive, and bits 5-1 give the tag number.
- high-tag-number form: Two or more octets. First octet is as in low-tag-number form, except that bits 5-1 all have value 1 Second and following octets give the tag number, base 128, most significant digit first, with as few digits as possible, and with the bit 8 of each octet except the last set to 1. If bit 6 has value 1, it indicates that the encoding is constructed.
- length (two forms):
- Short form: One octet. Bit 8 has value 0 and bits 7-1 give the length.
- Long form: Two to 127 octets. Bit 8 of first octet has value 1 and bits 7-1 give the number of additional length octets. Second and following octets give the length, base 256, most significant digit first.
- value: actual value as byte array of defined length.
Class encodings:
| Class | Bit 8 | Bit 7 |
|---|---|---|
| universal | 0 | 0 |
| application | 0 | 1 |
| context-specific | 1 | 0 |
| private | 1 | 1 |