## 2 Related Work
However the scheme does not address collusion between the authorities. In addition, a collusion of consecutive RSUs may reveal a vehicle's trajectory since primary pseudonym is always revealed when requesting for short term credentials. Finally, a corrupted CA may refrain from deleting users' VIDs (vehicle ID) and is able to impersonate any vehicle. Various protocols in the literature are based on the assumption of a fully trusted authority (TA). In [26] KMC is a trust authority and is fully trusted by all the other entities. Each vehicle is equipped with a tamper proof device. Again the KMC is aware of all sensitive vehicle and driver information and may impersonate any vehicle. In [27] trust is distributed among a fully trusted authority TA and the RSUs which are considered semi-trusted, lower level authorities. A corrupted TA may impersonate a vehicle and request temporary signing keys from an RSU. Location of the vehicle is periodically revealed to TA. The ECPP protocol presented in [28] can efficiently deal with the growing revocation list, while achieving conditional traceability. Location of the vehicle is revealed to the RSU when requesting for a short time anonymous key. Obviously, the TA may impersonate any vehicle and acquire a short-time anonymous key. Vehicle impersonation by the TA is also possible in PACP [29], where the TA may impersonate any vehicle in the communication protocol between the OBU and an RSU. Consequently it may also sign messages on behalf of any vehicle. Location of the vehicle is revealed to RSU. SPECS [30] and b-SPECS+ [31] are also vulnerable to impersonation attacks by the TA. In NECPPA [32] the location and the ID of a vehicle are revealed to the RSUs. In addition, the TA may again impersonate any vehicle in the OBU joining RSU phase. Similarly, in EAAP ([33]) and CL-CPPA [34] the TA may use the relevant information from the registration phase and impersonate any vehicle. In [35] a two-factor lightweight privacy preserving authentication scheme for VANET is presented. It relies on a fully trusted certificate authority CA that distributes trust in tamper-proof devices. The scheme depends on the correct use of tamper-proof devices, while the CA is aware of all sensitive vehicle and driver information and may impersonate any vehicle by simulating the function of the tamper-proof device. Batch verification schemes like [36, 37, 38, 39, 40] either allow authorities to impersonate any vehicle or reveal the current location of the vehicle to the corresponding RSU. Impersonation and framing attacks by dishonest or compromised authorities against vehicles is also possible in the schemes presented in [41, 42, 43, 44, 45]. Various recent schemes are subject to impersonation and framing attacks during the registration phase. For example, in [46, 47, 48] the TA acquires all relevant information of vehicles during registration and can impersonate or frame a vehicle. In [47] it is also assumed that RSUs will not collude with each other. In [49] consecutive RSUs may reveal the driver's path since a pseudo identity of a vehicle is known to the RSU. Although the real identity of the vehicle is not revealed during registration, impersonation attacks against vehicles are possible if the registration authority colludes with the RSUs. Very few works, like [50,51] provide security from framing attacks against vehicles. However in both these works the TA may impersonate any vehicle and request pseudo ID from the KGC. Then by creating a pseudo key \((x_{i},P_{i})\) the TA may sign valid messages using the target vehicle's credentials. Paper positioning and comparison with the related workTo the best of our knowledge, the proposed scheme is the first protocol that may protect vehicles from both framing and impersonation attacks by misbehaving authorities, without requiring the maintenance and distribution of revocation lists. Even a collusion of corrupted authorities will not be able to impersonate a vehicle. Thus security from such attacks is assured even for corrupted or compromised authorities.