tachograph

name	tachograph
description	specialized knowledge for parsing/marshalling EU tachograph files. Use when needing to understand the .DDD/.V1B file format, TLV/TV protocols, ASN.1 mappings, or EU regulation compliance.

EU Tachograph Domain Knowledge

This skill provides the domain expertise required to work with EU Tachograph files (Driver Card & Vehicle Unit).

Core Concepts

• Regulation First: All implementation must derive directly from EU regulations (Data Dictionary).
• Binary Fidelity: Support perfect round-trip parsing/marshalling (byte-for-byte exactness).
• Type Safety: Use specific types for specific generations/formats.

Workflows

1. Card File Structure (TLV)

• Architecture: See card_structure.md for TLV, DF/EF structure, and signature handling.
• Pattern: Use a "Two-Pass" parsing strategy (Raw TLV -> Semantic Structure).

2. Regulation & ASN.1

• Regulation: Consult regulation.md to locate specific chapters (Data Dictionary is critical).
• Definitions: Use ASN.1 definitions from the regulation as the source of truth for all data types.

3. Generations & Versions

• Details: See versions.md for a comprehensive overview of:
  • Generation 1: Legacy (SHA-1, RSA-1024).
  • Generation 2 V1: Smart Tachograph (SHA-256, ECC).
  • Generation 2 V2: Smart Tachograph with GNSS/OSNMA support.
  • Protocol Matrix: TLV (Card) vs TV (VU) tag ranges and structures.
• Detection: Use tag-based detection logic for both Card and VU files.

Best Practices for Implementation

Data Parsing

1. ASN.1 Compliance: Every parser function should reference the ASN.1 definition from the spec.
2. No Magic Numbers: Define constants for all binary offsets and lengths.
3. Exact Lengths: Validate fixed-structure lengths exactly (==), not loosely (>=).
4. Nil Handling: The protocol has no concept of "null", only "default values" (e.g., spaces for strings, 0 for numbers). Parsing functions should handle missing or zero-length input safely.

Data Modeling

1. Integers: Use signed integers (int32/int64) for general numbers; the protocol rarely uses unsigned types in a way that maps cleanly to uint in all languages.
2. Bitflags: Prefer boolean fields over integer bitmasks for clarity.
3. Signatures: Model signed EFs by including a signature field (unless Regulation forbids).