## 1 Introduction
However, there is still a lack of experimental deployment and comprehensive performance evaluation of these standards under realistic environments, traffic load and application use cases, especially regarding the security of vehicular communications and the safety of ITS applications. Even though ITS will have a great potential in the near future, there are many open research challenges and issues that need to be tackled in order to foster the emergence of safe ITS applications. Indeed, since this technology heavily relies on wireless communications, several threats and attacks can affect its functioning [12; 13; 14; 15] and, thus, compromise the safety of the involved ITS entities and users. Typical attacks on ITS include denial of service (DoS) attacks [13], Sybil attacks [16], man-in-the-middle attacks [17], eavesdropping [18] and many others. For example, several external attacks were successfully demonstrated [19] on a sedan vehicle that has an e-call application and that is widely used in the U.S. More recently, it was demonstrated that long-range attacks were also possible on real vehicles using malicious smartphones [20]. Consequently, several security requirements should be ensured to resist these attacks, including authenticity, confidentiality, integrity, non-repudiation and data trust, and, thus, to enable the emergence of future autonomous (or driverless) vehicles [21]. This paper aims to review the current research challenges and opportunities related to the development of secure and safe ITS applications and to provide a comprehensive study about existing security threats and their corresponding cryptographic countermeasures. In this context, it is important to highlight the main differences of this paper in comparison to other surveys on ITS security, such as [12; 13; 14]. In contrast to existing works, this paper evaluates all recent cryptographic-based solutions that have been proposed separately for each ITS threat. To the best of our knowledge, most research works on the ITS security domain were either papers that address specific problems or general surveys. There is no previous work that focuses on linking ITS security issues with related cryptographic techniques, which can entirely solve or reduce problems and their impact. Moreover, an ITS safety application case study is analyzed in detail and evaluated in light of the European ETSI TC ITS standard. An experimental test-bed and implementation of this ITS standard is presented, and various elliptic curve digital signature algorithms (ECDSA) are evaluated for signing and verifying ITS safety messages. To the best knowledge of the authors, there have been very few works [22; 23] that have attempted to evaluate the ETSI ITS security architecture. The first implementation was described in [22], and some weaknesses in the early version of the standard were identified and discussed; whereas some preliminary cryptographic performance indicators were presented in [23]. Finally, lessons learned, open research challenges and opportunities are discussed. The remainder of this paper is structured as follows. Section 2 explores the architecture model, the main characteristics and challenges, as well as the target applications of intelligent transport systems. Section 3 surveys the main ITS standardization activities in Europe and the U.S. and briefly surveys the most important ITS projects that have been developed in the past and that are currently active. Section 4 discusses the main ITS security requirements and threats and classifies them based on their respective cryptographic countermeasures. Section 5 analyzes in detail an example of an ITS safety application use case (_i.e._, the collision risk warning application) in light of the European ETSI TC ITS standard. An implementation of the ETSI TS 103 097 [24] security standard is described, and several elliptic curve digital signature algorithms (ECDSA) are evaluated for signing and verifying ITS safety messages.