# An Enhanced Privacy-Preserving Authentication Scheme for Vehicle Sensor Networks
## 2 Related Work  
To cope with the challenges associate with VANETs, many types of authentication schemes have been investigated. [PERSON] et al. [6] introduced a two-phase authentication protocol for sensor networks that uses certificates and consequently cannot preserve the unlinkability of messages. [PERSON] and [PERSON] [7] proposed an authentication scheme for VANETs using anonymous certificates, in which each vehicle can utilize distinct key pairs in each authentication stage to avoid being tracked. However, frequent changing of key pairs is likely to result in burdensome management and storage requirements. [PERSON] et al. [8] proposed an alternative way to avoid the complexity of preloading a large number of anonymous certificates with the support of RSUs. When a vehicle passes an RSU, it will be issued a short-term anonymous certificate; thus, the unlinkability of messages is preserved. However, the efficiency will inevitably be low because each vehicle must frequently interact with RSUs. Subsequently, [PERSON] et al. [9] introduced another secure scheme that does not require interaction with RSUs, in which membership managers, rather than RSUs, are responsible for the issuing of certificates based on group signatures. However, the efficiency of this solutions is low. [PERSON] et al. [10] presented two additional authentication schemes with privacy preservation; however, the computational costs of their methods are somewhat high because of the utilization of bilinear pairing. Similarly, [PERSON] et al. [11] introduced an authenticated key agreement scheme based on bilinear pairing. [PERSON] et al. [12] later showed that [PERSON] et al.'s scheme is susceptible to impersonation attacks, and proposed a more secure authenticated key agreement scheme; however, the computational cost of this scheme is again somewhat high because of the utilization of bilinear pairing. In addition, an authentication scheme with access control for VANETs was investigated by [PERSON] et al. [13]; however, [PERSON] et al. [14] later showed that [PERSON] et al.'s scheme [13] is susceptible to privilege escalation attacks.  
Recently, [PERSON] and [PERSON] [15] developed a trust-extended authentication mechanism, called TEAM, for VANETs. In TEAM, vehicles are classified into three types, namely, law executors (LEs), mistrusted vehicles (MVs) and trusted vehicles (TVs), as shown in Figure 1. Moreover, it required each vehicle is equipped with a tamper-proof device from which no attacker can extract any stored data, which is so strong that it is not practical. The performance of this mechanism in response to several types of attacks has been analyzed; however, the linkability of messages in the authentication procedure and the possibility of internal attacks during the secure communication procedure, which can easily be executed by a malicious vehicle, have been ignored. A malicious vehicle can trace a driver by intercepting the message sent during the authentication procedure because the values \(D_{i}\) and \(M_{4}\) are constant. Moreover, a malicious trusted vehicle can compute the real identity of a user and the session key by intercepting a message communicated via the secure communication procedure because it possesses the authorized parameter. [PERSON] et al. [16] proposed an enhanced trust-extended authentication scheme based on TEAM. However, their scheme fails to protect against internal attacks. Therefore, we have developed an improved authentication procedure and secure communication procedure and have proven their correctness and security. The updating of the constant values used in the authentication procedure is performed by the user himself. Finally, we analyze the computational costs and security features of the improved secure communication procedure.