A Vehicular Ad-hoc NETwork or VANET is a form of Mobile Ad-hoc NETwork or MANET which provides communication between vehicles and between vehicles and road-side base stations. A vehicle in VANET is considered to be an intelligent mobile node capable of communicating with its neighbors and other vehicles in the network. VANET is different from MANET due to high mobility of nodes and the large scale of networks. Security and privacy are the two main concerns in designing a VANET. Although there are many proposed solutions for improving securities in VANET but security still remains a delicate research subject.