## 7 Review of Security and Privacy-Preserving Schemes in VANETs  
### Pseudonyms Coupled with PKI-Based Schemes  
As the vehicles are transmitting their location information, e.g., to warn other users of accidents, researchers [16, 17] have discussed using pseudonyms to preserve the privacy of users. The use of Pseudonyms (PNs) hides the real identity of the user and prevents the linking of the real identity of the user with the pseudonym, thereby providing unlinkability. However, the Regional Trusted Authority (RTA) that issues pseudonyms can reveal the real identity of the user if the user commits an illegal act, hence providing traceability. In all cases, the On Board Unit (OBU) is assumed to be a tamper-proof device which contains the unique Vehicle Identification Number or VIN. This VIN is tied to an identity certificate at the time of registration. From here on, the Trusted Authority (TA) (e.g., transport authority) is tasked with issuing new blocks of pseudonyms/certificates to the vehicles, to be used while communicating with the Road Side Units (RSUs). These certificates have a validity period and the vehicle switches its pseudonyms to ensure privacy. The authors in [18] suggest that short-term linkability should be allowed so that the receiver is able to verify that two or more messages in a short time frame have come from the same sender, as this might be required by applications. The recommended time after which the pseudonyms are switched also varies, e.g., every minute or every few minutes, with every message or every second [19]. However, the frequency of PN changes is directly proportional to the computation overhead at the RSU, which can slow down packet delivery. Also, the OBU is also supposed to store the public keys of all CA (Certificate Authority) / TA. Furthermore, different PKI-based techniques that use a CA/TA connected with the RSU have been suggested. The downside of the pseudonym scheme is that the user (OBU) has to have hundreds or thousands of these pseudonyms stored on board and they need to be refreshed after some time (e.g., once a year). Different papers have explored this and proposed different methods of refreshing these, e.g., downloading new pseudonyms when the vehicles go in for service (once a year). It is also suggested that the pseudonyms are refilled at social spots [20]. In [13], a PN distribution scheme is proposed which incorporates control channels and service channel intervals in communication between vehicle and RSU for PN refill. It is important to note that pseudonym schemes alone do not support authentication, integrity, and non-repudiation. Signing protocols have been proposed that provide integrity and authentication. In this, a large number of certified public-private key pairs are stored in the OBU. Each key pair will be used for a short period of time and then discarded. Using PKI involves downloading and maintaining public key certificates, which results in heavy computational overhead that is not desirable in VANETs as the computing power of OBU is limited. Anonymous certificates in PKI-based schemes can guarantee identity privacy but cannot guarantee location privacy. This is because an attacker can monitor the certificate change by a vehicle between two observation points while moving in the same lane with the same speed. Also, the TA/CA has the ability/capability to identify the real identity of a vehicle based on its anonymous certificate. In [15], a method has been proposed so that multiple authorities are required to de-anonymize a user to increase the security. In [17], authors propose that the users change their PNs only at predetermined locations (mix zones) where the density of vehicles is high and the speed and direction of vehicles changes often. However, the authors conclude that such a technique provides limited privacy due to the inherent lack of randomness in vehicle mobility. A major drawback of using PN is maintaining the Certificate Revocation List (CRL), which keeps track of revoked certificates of misbehaving vehicles.