SKILL.md
ASN.1, BER, and DER
This skill provides reference material for Abstract Syntax Notation One (ASN.1) and its encoding rules (BER/DER).
Quick Reference
Universal Tags
| Tag (Hex) | Type |
|---|---|
01 |
BOOLEAN |
02 |
INTEGER |
03 |
BIT STRING |
04 |
OCTET STRING |
05 |
NULL |
06 |
OBJECT IDENTIFIER |
10 |
SEQUENCE / SEQUENCE OF |
11 |
SET / SET OF |
13 |
PrintableString |
14 |
T61String |
16 |
IA5String |
17 |
UTCTime |
Identifier Octet (First Byte)
Bits 8-7: Class
00: Universal01: Application10: Context-specific11: Private
Bit 6: PC (Primitive/Constructed)
0: Primitive (Value is directly in contents)1: Constructed (Contents are nested TLV elements)
Bits 5-1: Tag Number (if < 31)
Detailed Guide
For a comprehensive guide on notation, encoding rules, and specific type handling, read the bundled reference:
references/guide.md
Contents of the Guide:
- Introduction: Overview of OSI, ASN.1, BER, DER.
- ASN.1 Notation: Simple types, structured types, tagging.
- BER: Primitive vs. Constructed, Definite vs. Indefinite length.
- DER: subset of BER for unique encoding (Canonical).
- Type-Specific Encodings: Detailed breakdown of how to encode INTEGER, BIT STRING, SEQUENCE, etc.
- Example: Walkthrough of an X.500 Name encoding.
Common Operations
Deciphering a Tag
If you encounter byte 0x30:
- Binary:
0011 0000 - Class:
00(Universal) - P/C:
1(Constructed) - Tag:
10000(16 -> SEQUENCE)
If you encounter byte 0xA0:
- Binary:
1010 0000 - Class:
10(Context-specific) - P/C:
1(Constructed) - Tag:
00000(0 -> [0])
Variable Lengths
- Short Form:
0x00-0x7F(0-127 bytes). - Long Form: Bit 8 is 1. Bits 7-1 = number of length bytes following.
- Example: 0x81 0x80 -> Length is in next 1 byte (0x81), value is 0x80 (128). - Example: 0x82 0x01 0x00 -> Length is in next 2 bytes (0x82), value is 0x0100 (256).