## 1 Introduction  
A general VANET has a three-tier structure [1], which comprises a trusted authorizer (TA), many road side units (RSUs), and vehicles. The TA is the central trust tier, and it is connected to the RSU via a wired network. The communication between the RSUs and vehicles uses the wireless communication protocol IEEE 802.11p. IEEE 802.11p is a revision of 802.11 with the addition of Wireless Access in the Vehicular Environment (WAVE) [2]. All RSUs (second tier) and vehicles (third tier) must register with the TA to obtain initial certification, identities or common secrets to enable them to make requests anonymously. RSUs are installed at the side of the road to help vehicles maintain anonymity and authenticate messages. The vehicles can broadcast, exchange, or receive messages about road conditions, traffic conditions, their positions, or their speed to avoid accidents and worsening traffic jams. Malicious attackers [3; 4] may collect transmitted messages in VANETs to obtain the private information of users. To resist malicious attacks and maintain the privacy of the vehicles, each vehicle must remain anonymous, and its messages must be sent anonymously, so the authentication of messages in VANETs is an important issue.  
In a VANET, communications can be classified into two types--without RSU and with RSU. In communication of the first type, each vehicle broadcasts messages to other vehicles or communicates confidentially with specific vehicles. In this scenario, vehicles must ensure their privacy, their confidentiality and the authentication of messages by themselves. In the second scenario, an RSU supports the privacy and confidentiality of vehicles, and message authentication.  
In a VANET, messages are authenticated to ensure that received messages are valid and have been sent by a legal source. To preserve privacy, the real identity of a vehicle cannot be exposed or traced. In this paper, the concept of random key pre-distribution that is used in Wireless Sensor Networks (WSN) is modified to random secret pre-distribution (RSP) that integrates identity-based cryptography (IBC), to build an environment in which vehicles can maintain anonymity, communicate confidentially, authenticate messages, and resist malicious attacks with the assistance of RSUs or by themselves in a sparse RSU environment. In the proposed scheme, the TA maintains a large pool of secrets that will be pre-distributed randomly to all RSUs and vehicles as the original registration set. All RSUs have a pseudo random generator (PRNG) and the same seed value that is provided periodically by the TA, so they have the same secret pool from which to issue the randomly selected secret to vehicles. Based on the common secrets held in common either between RSUs and vehicles or among vehicles under an RSU, entities of both types can authenticate each other, authenticate messages, and communicate confidentially following the pairing process. The proposed scheme satisfies the security requirements of a VANET, including message authentication, identity verification, non-repudiation, confidentiality, conditional anonymity, and un-traceability.  
In this paper, Section 2 introduces related works and techniques that are used herein. Section 3 describes the proposed schemes. Section 4 analyzes the security and performance of the proposed schemes. The final section draws conclusions and provides suggestions for future works.