Abstarct: Nowadays, vehicular ad hoc network has been proposed as a comprehensive and new plan, which aims to provide road safety, traffic management and convenience applications for drivers and passengers on the road. In this network, communications are in two ways: vehicle to vehicle and vehicle to Infrastructure (road side unit). Different messages containing events warning about road and traffic conditions, traffic information, private information of vehicles such as speed and location are exchanged in these communications. Unfortunately, wireless communications and the high-speed mobility of a large number of vehicles pose many challenges in VANETs. It is evident that any malicious behavior of users (vehicles), such as injecting false information, modifying and replaying the disseminated messages, discarding routing packets in the network and impersonation has irreversible effects on people's lives. In addition, users show prime interest in protecting their private information leading to unique identification in the network. So, it is clear that security and privacy preservation are two critical challenges for VANET deployment in real world. Studies have shown that Public Key Infrastructure (PKI) is a well-recognized solution for secure VANET communications. Consequently, in this thesis, we explore the breaches addressed in this infrastructure and examine two important security requirements. In the first problem, positioning is investigated. Valid information about the location of the vehicle is a basic requirement for a large number of VANET applications. Disseminating fake position information generated by malicious vehicle or malfunctions of vehicles location finding system (e.g. GPS), can disrupt traffic management applications and so cause to congestion and road accidents. Therefore, we have tried to improve a localization scheme baxsed upon the information provided by road side units that have not limitations of the GPS-baxsed systems. In this improvement, we change road side unit deployment such that has two advantages: first, location finding now involves less communication overhead, second, in validating a vehicle's location the large state space due to malicious vehicle now gets significantly smaller. In the second problem the Sybil attack is discussed. In such attack, the malicious vehicle impersonates other vehicles or introduces fake identities in the network. Its mission is to interrupt a voting system, routing method and reducing efficiency. In this dissertation we first simulate the Sybil attack on three different routing algorithms, and then by analyzing various defensive approaches we select the best one that has the public key infrastructure. Finally using a suggested protocol for this matter which has a 100% detection rate, firstly we reform this approach and then simulate it, in terms of scalability and practical implementation.