Threat Analysis of Seed/Key Authentication in Automotive OBD Diagnostic Protocols: Attack Vectors and Cryptographic Countermeasures

The On-Board Diagnostics (OBD) interface, standardized across modern vehicles through ISO 14229 (UDS), SAE J1939, and related protocols, presents a critical and frequently underestimated attack surface in automotive cybersecurity. Central to access control within these protocols is the seed/key authentication mechanism — a challenge-response scheme designed to restrict privileged diagnostic sessions to authorized entities. Despite its widespread deployment across hundreds of millions of vehicles, seed/key implementations remain highly heterogeneous, frequently relying on proprietary algorithms whose security rests on the wrong foundation: the secrecy of the algorithm itself rather than the secrecy of cryptographic key material. This paper presents a structured threat analysis of seed/key authentication in automotive OBD diagnostic contexts, applying a methodology aligned with ISO/SAE 21434 Threat Analysis and Risk Assessment (TARA). We introduce the distinction between algorithm secrecy and key secrecy as a central analytical lens, apply it to characterize attack vectors including brute force, reverse engineering, replay attacks, and ECU memory extraction, and evaluate the adequacy of cryptographic countermeasures currently deployed in the industry. Based on this analysis, we propose the Seed/Key Security Assessment Framework (SKSAF), a practical four-phase methodology for evaluating seed/key implementations in multi-brand diagnostic environments.