In order to solve the problem of illegal member's tracking attack, which caused by the vehicle units' privacy disclosure in vehicular ad hoc networks (VANETs), a vehicle identity authentication protocol based on lightweight group signature was proposed by analysis of topology and communication characteristics of VANETs in this paper, which can authenticate the vehicles anonymously in a fast and efficient way. The protocol has five stages. In the initialization phase, the public/private key pairs and system parameters of the group were generated by the VANETs system, then the group public key and system parameters were distributed to the on-board units by the roadside auxiliary facilities. The group private key was kept by the group manager. When a vehicle unit entered VANETs, the unit's own identity was submitted to the group manager by the blind signature. A group certificate would be distributed to the vehicle unit by the group manager when authentication passed. In the cooperative communication stage, the vehicle member who owned the group certificates signed the state information with the valid certificate and group public key, then sent it to the nearby vehicle units by the car sensors, and achieved cooperative driving with surrounding vehicles. In the message verification stage, only can the legal vehicle members open the received status information by using group public key, but couldn't know the true identity of the message sender. In this way, the anonymous communication among vehicles was realized. In the stage of signature verification, when a vehicle unit broadcasted a false message for the purpose of exclusively using road resource and caused traffic accident, the group manager can open the signature of the message by using the group private key, and traversed the corresponding vehicle members to carry on the accountability. The innovation of the paper was the usage of improved lightweight group signature technology, which could ensure that the length of group public key and group signature didn't depend on the number of group members. Zero knowledge proof was also used as a means of membership authentication which improved the speed of authentication among the members. The security of the protocol was analyzed and proved mathematically in this paper, and a LAN simulation platform composed of 100 PC machines was built to simulate the cooperative communication among vehicle units in VANETs. The experimental results showed that authentication time of the protocol was about 7 ms among 100 vehicle users. The performance of the proposed protocol is superior to the contrasted schemes. It greatly reduced the storage and calculation burden of the vehicle units during the process of identity authentication.
