Vehicle-to-vehicle (V2V) and vehicle-to-roadside (V2R) communications have raised a series of challenges due to data security and user privacy. Vehicular ad hoc networks (VANETs) have become the target of malicious attacks. Authentication has been widely adopted in VANETs to guarantee secure communications. However, existing authentication schemes have failed to meet stringent time requirements of VANETs leading to information loss. In this paper, we propose an efficient anonymous batch

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... cation (EABA) scheme aiming to reduce the message loss rate of vehicles and roadside units (RSUs). In the EABA scheme, we introduce a message classification algorithm and allow receivers to authenticate messages based on the classification result so that messages with high priority will be authenticated preferentially. In addition, we propose a cooperative authentication mechanism, where RSUs and vehicles share authenticated messages to reduce the workload on RSUs when the network is saturated. The proposed method is evaluated by extensive simulations, and the results demonstrate that the EABA scheme can reduce the message loss significantly and guarantee information security.