How Secure Are Our Roads? An In-Depth Review of Authentication in Vehicular Communications

Intelligent transportation systems are pivotal in enhancing road safety by enabling intra-vehicle communication. Due to the nature of the wireless communication link, several potential risks of attacks exist, including impersonation, modification, and replay. To ensure the security of vehicular ad hoc networks (VANETs) against malicious activities, secure data exchange between inter-communicating terminals, specifically vehicle-to-everything (V2X) communication, becomes a critical technological challenge that requires attention. Existing authentication methods for VANET applications mainly rely on crypto-based techniques. The emergence of physical (PHY)-layer authentication has gained prominence, leveraging the inherent characteristics of wireless channels and hardware imperfections to distinguish between wireless devices. The PHY-layer-based authentication is not a standalone alternative to cryptographic methods, but it shows potential as a supplementary approach for re-authentication in VANETs, referred to as “cross-layer authentication”. This comprehensive survey thoroughly evaluates the state-of-the-art of crypto-based, PHY-layer-based, and cross-layer-based authentication methods in VANETs. Furthermore, this survey delves into integrating different sixth-generation (6G) and beyond technologies, such as reconfigurable intelligent surfaces (RIS) and federated learning, for enhancing PHY-layer authentication performance in the presence of active attackers. Furthermore, in-depth insights into the advantages of cross-layer authentication methods are presented, along with exploring various state-of-the-art VANET security techniques. A detailed technical discussion is provided on these advanced approaches, and it is concluded that they can significantly enhance the security of intelligent transportation systems, ensuring safer and more efficient vehicular communications.