On the security of aggregate signature–based conditional privacy-preserving authentication schemes for VANETs

Certificateless aggregate signature (CLAS) schemes enable multiple vehicles to combine their individual signatures into a single compact authenticator, significantly reducing transmission and verification overhead in bandwidth-constrained vehicular ad-hoc networks (VANETs). This efficiency makes CLAS particularly attractive for real-time authentication in large-scale intelligent transportation systems (ITS). However, we show that two representative CLAS-based conditional privacy-preserving authentication schemes, proposed by Zhu–Guan and Chen–Guan, are vulnerable to a Type I adversary capable of replacing a vehicle’s public key. Such an adversary can forge valid signatures on arbitrary messages without obtaining the vehicle’s partial private key due to the lack of cryptographic binding between public keys and ephemeral randomness, which allows algebraic elimination of the master public key during verification. We analyze the root causes of this flaw and propose hash-based countermeasures that enforce strong binding among all public components. Our results further reveal that conventional linear hash-dependent CLAS constructions face inherent limitations in balancing security, ultra-low latency, and scalability. Thus, a next-generation CLAS design paradigm is required one that moves beyond traditional linear architectures to achieve deployment-level performance in current 5G-V2X environments while being inherently scalable toward future 6G-enabled intelligent transportation systems.