Privacy-Preserving Certificate Linkage/Revocation in VANETs Without Linkage Authorities

Vehicular communication (V2X) technologies are expected to become common in the future, providing better efficiency and safety in transportation. This envisioned large-scale deployment, however, critically depends on addressing some issues. In special, to prevent abuse by drivers, messages exchanged among authorized vehicles must be authenticated. This implies the need of a Vehicular Public Key Infrastructure (VPKI). Unlike traditional PKIs, though, VPKIs are also expected to preserve the privacy of honest drivers, preventing their vehicles from being easily identified or tracked. One promising VPKI solution, which copes with such requirements and is among the main candidates for standardization in the United States, is the Security Credential Management System (SCMS). In this paper, aiming to enhance and address shortcomings identified in SCMS, we provide two main contributions. First, we describe and fix two birthday attacks against SCMS's certificate revocation process, thus improving the system's long-term privacy. Second, we propose a method that simplifies SCMS's architecture, removing the need for Linkage Authorities (LAs); this approach cuts down deployment costs while reducing the system's attack surface, in particular against some troublesome forms of replay attacks that are hereby unveiled.