# A More Efficient Conditional Private Preservation Scheme in Vehicular Ad Hoc Networks
## 1 Introduction  
Many people are seriously injured or killed in road traffic accidents due to carelessness, traffic congestion, traffic violations, inadequate road information, increased population and lack of secure infrastructure. Therefore, reducing traffic congestion and enhancing road safety are the issues that many people are most concerned about. With the development of automotive technology and wireless communication technology, VANETs have aroused widespread interest. In VANET, smart vehicles equipped with the on-board device can be connected to each other and surrounding infrastructures easily [1]. Any vehicle can send other nearby vehicles about the traffic and road conditions to warn of potential emergencies and traffic jams. In addition to helping prevent accidents, VANETs also provide convenience and business services that will help improve a driver's experience [2]. However, before taking this wonderful application into practice, several obstacles in VANETs need to be addressed: delay, service cost, security and privacy. It is worthwhile noting that security and privacy issues are becoming more and more import in many people's lives [3]. So far, the security issues of VANETs have been studied in detail, while the privacy issues still have many open questions [4; 5; 6]. In the absence of privacy protection, the adversary can track the location of the target vehicle by collecting their routine information. Even worse, if a legitimate anonymous vehicle becomes malicious, there is no way to identify and revoke it. Thus, it is necessary to limit malicious vehicles, the privacy protection must be conditional for the vehicles which are able to be tracked and revoked if need. In addition, high mobility of vehicles and frequently changed topology are the other important characteristic of the vehicular network. Therefore, it is very challenging to design an efficient privacy preserving anonymous authentication scheme for VANETs. In the past few years, many secure VANET schemes have been proposed, but there are still some unsolved problems. In [7], it is stated that distributing and searching of a huge certificate revocation list (CRL) is inevitable. The overhead of authentication will increase linearly with the increase of CRLs. The higher overhead of identifying and revoking malicious vehicles makes group signature and identity-based signature (GSIS) [8] and hybrid method [9] unsuitable for real-time VANETs. Efficiency conditional privacy preservation (ECPP) [10], proposed by [PERSON] et al., is a relatively practical scheme which deals with the growing revocation list while achieving conditional traceability by the authorities, but it also suffers drawbacks: (1) It takes more latency time for sending and verifying the certificate and signature which are not efficient for the high speed movement of vehicles; (2) It needs large space to storage every vehicle's temporary information to reveal the malicious vehicles when the tracking phases; (3) Vehicle will interact with infrastructure unit several times during short-time anonymous key generation. To resolve these problems, we propose a more efficient conditional private preservation scheme based on ECPP. The main contributions of this paper include the following: (1) Reducing the storage space. When dispute occurs, the centralized Trusted Authority can decrypt the real identity of the rogue vehicles just by certification. So it does not need to storage temporary information, and that will save considerable storage space. (2) Lowering down half of the interaction steps during anonymous key generation phase. When vehicles move along the road, the speed is usually high, and it needs fast interaction. Fewer interaction steps help to increase the interaction speed (3) Decreasing the computational cost and communication overhead for sending and verifying the certificate and signature. The presented performance studies and comparisons with ECPP and other related schemes demonstrate that this scheme is effective and efficient.