## 2 Related Work  
Security in vehicular networks was initially discussed in [5, 14]. Since then, reducing the confidence level on trusted authorities is an open and challenging problem. Various schemes have been proposed in the literature, attempting to weaken the trust assumption for the authorities, either by distributing the trust among different authorities or by considering semi-trusted authorities. In various schemes vehicles are equipped with Event Data Recorders (EDR) that keep a copy of the communication. Unframeability is assured under the assumption that EDRs are trusted hardware devices. However, impersonating a vehicle is still in question, since VANET schemes offer conditional traceability and only misleading messages are the object of investigation. In the literature, various group signature schemes have been proposed [15, 16], to allow users to anonymously sign messages on behalf of a group. These schemes provide strong unframeability properties, even in the presence of corrupted authorities. Although group signatures have been widely used in VANETs for anonymous authentication (e.g., [17, 18]), they require maintaining a Certificate Revocation List (CRL), which leads to long computation delay and consequently high message loss [19]. In addition, re-issuing of keys may be required, when users diverge from honest protocol execution. Location privacy is the capability of preventing a third party from knowing the present and past location of the vehicle in the network [20]. Thus not only the current location is private (location point privacy protection) but also the trajectory of the vehicle is kept secret (trajectory privacy protection). In the following paragraphs we will briefly review existing privacy preserving communication protocols aiming at location privacy for vehicles, with an emphasis on their security level against vehicle impersonation and framing attacks. A V2I communication protocol is presented in [21]. Trust is divided in two trusted authorities, the tracing authority (TRA) and the private key generator (PKG). However a collusion of TRA and PKG may impersonate a vehicle. A scheme based on a semi-trusted authority is presented in [22]. Trust is divided in two authorities; the trusted data center (TDC) who is responsible for managing the real identities of the vehicles and is fully trusted and the semi-trusted management center (STA), who is responsible for managing RSUs and vehicles. To generate a partial key the current and previous location of the vehicle is revealed to the STA. All the IDs and the partial keysof the vehicles are known to the STA. Thus if corrupted, the STA may impersonate a vehicle, create new vehicle identifiers and sign messages on their behalf. In the scheme of [23], a fully trusted authority TA assigns a real identity RID and a password PWD for each vehicle and pre-loads \(\{RID,PWD\}\) into its tamper-proof device. TA may impersonate any vehicle by simulating the function of the tamper-proof device. In [24] a certificateless signature scheme for VANET is presented. The scheme is based on two authorities that are considered as fully trusted: the regional transport authority (RTA) and the key generation center (KGC). An \(RSU_{j}\) takes as input the \(\mathcal{Q}_{ID}\) of the vehicle and generates a corresponding pseudonym \(PS_{j}\). Although the network is divided in autonomous sub-networks, consecutive RSUs may reveal vehicle's trajectory. In addition, if the authorities are compromised they may forge and/or frame any vehicle. A compromised KGC may impersonate any vehicle and request from any \(RSU_{j}\) a pseudonym \(PS_{j}\), while a compromised RTA may impersonate any vehicle and request a partial private key from KGC using vehicle's \(ID\). In [25] a hierarchical privacy preserving pseudonymous authentication protocol for VANET is presented. The scheme is based on two honest-but-curious authorities, the certification authority CA and the revocation authority RA. Authentication of the vehicles is based on long and short term credentials.