# A More Efficient Conditional Private Preservation Scheme in Vehicular Ad Hoc Networks
## 2 Related Work  
There have been many research works about anonymity authentication of VANETs in the last past years. In all existing works, pseudonym-based authentication schemes are the major approaches [6; 7; 11; 12]. These pseudonym-based approaches can perfectly achieve privacy preservation, but the main limitation is that the TA is needed to keep the pseudonyms of each vehicle in storage space and required to change them in a frequent manner. To overcome this limitation, [PERSON] et al. [13] proposed a conditional privacy preserving authentication approach using anonymous certificates. However, it is not efficient because of frequent interactions with infrastructures. [PERSON] et al. [10] developed an efficient conditional privacy preservation scheme. This scheme can achieve perfect conditional privacy with bilinear pairing technology, but it still faces some limitation that takes more time with infrastructures and needs large storage space to save temporary messages. \begin{table}
\begin{tabular}{l l} \hline \hline
**Notation** & **Descriptions** \\ \hline \(OBU\) & The smart vehicle equipped with on-board unit \\ \(RSU\) & Road side unit \\ \(TA\) & The centralized Trusted Authority \\ \(L_{j}\) & The location of RSU \\ \(ID_{i}\) & The real identity of vehicle \(i\) \\ \(PID_{i}\) & The pseudo-id of vehicle \(i\) \\ \(Cert_{i}\) & The short-time certificate of vehicle \(i\) \\ \(X||Y\) & Concatenate operation \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Notations which are used in the paper. [PERSON] et al. [14] gave a privacy protection scheme for VANET based on an ID-based ring signature without tracking function, hence this scheme does not achieve conditional privacy. Then, [PERSON] et al. [5] proposed another ID-based scheme named CPPA, but it is vulnerable to replay and non-repudiation attacks. To overcome the drawback of ID-based schemes, [PERSON] improved a new efficient IBS scheme providing resistance against impersonation attack. However, it suffers to the modification attack which was demonstrated by [PERSON] et al. [15]. In 2015, [PERSON] et al. [16] introduced an anonymous authentication scheme. High computational overhead is the main limitation of [PERSON]'s scheme. In 2018, [PERSON] et al. [4] proposed a message recovery authentication scheme based on certificateless signatures. This scheme achieves low communication costs and computation overhead without the bilinear pairing operations. However, this solution is one-way communication only from the vehicle to the RSU, and the vehicle cannot receive traffic information transmitted from the center. Recently, [PERSON] et al. [17] proposed an energy efficient routing protocol for VANET, called Gree AODV, which focus on selecting the most efficient routing path between source and destination. It estimate the total power consumption and locate the lowest energy consumption route. In order to guarantee user satisfaction, a Trusted Third Party (TTP) cloud entity and Quality of Experience (QoE) game model has been proposed [18]. TTPs, which provide an abstraction layer between the vehicular service users and providers, are well-known profitable commercial organizations that provide and sell service to users. TTPs thus will simplify the process of resource discovery and selection in a smart city. Based on a cluster-based Trusted Third Party (TTP) model, [PERSON] et al. [19] propose a continuous availability scheme for diversified cloud services targeting vehicle cloud users. In this scheme, timely and successful receipt of information were used to reduce highway accidents by providing early warning messages to nearby and distant vehicles, thereby increasing response time to emergencies. Other studies accessed the anomaly detection schemes which can be used for data analysis, monitoring the normal behavior of road-side infrastructures and protecting the vehicle from potential attack [20]. [PERSON] et al. [21] presented several anomaly detection schemes to identify possible network intrusions.