An Improved Certificateless Aggregate Signature Scheme for Vehicular Ad-Hoc Networks

Vehicular ad-hoc networks (VANETs) can enhance traffic efficiency and improve traffic environment, but it faces security threats such as tampering and forgery during message transmission. In addition, limited bandwidth is also a major issue for VANETS. Certificateless aggregate signature (CLAS) is considered as one of the key techniques to solve these problems. CLAS can compress multiple signatures into a short signature, greatly reducing transmission bandwidth and verification overhead of signatures. Very recently, Kumar and Sharma proposed a CLAS scheme for VANETs and claimed that their CLAS scheme can resist forgery attacks. In this paper, we show that their CLAS scheme is insecure against coalition attacks. The first type of attack comes from internal signers. The second type of attack comes from the collusion between a malicious key generation center (KGC) and a roadside unit (RSU). Furthermore, we propose an improved CLAS scheme to withstand these attacks. The analysis results show that the validity of an aggregate signature can ensure the validity of each individual signature participating in the aggregation. Hence, our CLAS scheme is more suitable for bandwidth-constrained VANETs.