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

Identifier Octet (First Byte)

Bits 8-7: Class

• 00: Universal
• 01: Application
• 10: Context-specific
• 11: 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:

1. Introduction: Overview of OSI, ASN.1, BER, DER.
2. ASN.1 Notation: Simple types, structured types, tagging.
3. BER: Primitive vs. Constructed, Definite vs. Indefinite length.
4. DER: subset of BER for unique encoding (Canonical).
5. Type-Specific Encodings: Detailed breakdown of how to encode INTEGER, BIT STRING, SEQUENCE, etc.
6. 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).