## 6 Research Challenges and Opportunities  
Intelligent transport systems have been an active research area in recent years with great focus. However, there are still a few challenges to be overcome before mass market penetration and deployment of the V2X communications technology. First, existing ITS systems and standards still have a static selection of the security features. For instance, the ETSI TC ITS standard [24] recommends the usage of the ecdsa_nistp256_with_sha256 digital signature schema for signing and verifying safety messages (_i.e._, DENM and CAM), and the ecies_nistp_256 (_i.e._, Elliptic Curve Integrated Encryption Scheme with NIST P-256 curve) public key algorithm and the aes_128_ccm (_i.e._, Advanced Encryption Standard counter with Cipher Block Chaining Message Authentication Code and a block length of 128 bits) symmetric algorithm for the encryption and decryption of sensitive data. However, as already highlighted in our previous case study (_cf._ Section 5), existing security schema are based on expensive cryptographic overheads and are not able to efficiently handle a large amount of secure messages, without impacting the system's critical latency and, thus, the safety of ITS applications. There is therefore a need for novel ITS security frameworks that can dynamically adapt the security features at runtime, based on context changes [101] (e.g., local network density, wireless channel conditions, available resources, _etc._) and/or based on required quality of services (e.g., maximal end-to-end latency, packet delivery rates, _etc._). Such security frameworks could improve the scalability and safety of ITS systems at the cost of lower security overheads. Another way to achieve an important speedup in handling secure V2X communications is to delegate all of the cryptographic operations to dedicated hardware security modules (HSM) or trusted platform modules (TPM). It is expected that vehicles' OBUs will be equipped with such hardware modules, as security co-processors. As highlighted in our previous case study, the usage of higher CPU frequencies can enable the handling of a higher number of cryptographic operations, such as ECDSA signature verifications. However, the security gain of such an approach is still unclear, and more experimental investigations are needed to better quantify the benefits of such solutions. Second, the optimal broadcasting of secure ITS safety messages (or cooperative awareness messages) still continues to represent an important research challenge. Indeed, most of the road safety applications rely on these periodic beacons to construct local maps about the surrounding vehicles in order to enable the timely detection of collisions and/or road accidents. However, the underlying IEEE 802.11p layer is not efficient enough to handle a huge amount of transmissions due to collisions and interference, especially in high density networks. These collisions can lead to multiple packet re-transmissions, thus increasing the system's end-to-end latency. Moreover, the extra security overheads, in terms of packets sizes and security processing times, can also negatively impact the achieved quality of service. In this context, novel message broadcasting techniques are needed [102; 103; 104], such as the verify-on-demand (VoD) approach [103], which was proposed for IEEE 1609-based ITS systems and where signed messages are only verified on demand (e.g., based on their impact on the driver's safety). However, the integration of such techniques in ETSI TC ITS-based systems requires further research to better quantify their benefits in terms of achieved safety _versus_ security and QoS. Finally, the safety, security and QoS issues are generally considered as separate aspects of ITS systems and are evaluated independently of each other. In contrast, we believe that there is still a lack of research addressing the development of ITS applications by jointly considering the interplay between safety, security and QoS to limit potential threats to connected vehicles.