# Protecting Hybrid ITS Networks: A Comprehensive Security Approach
## 5 Conclusions  
C-ITS/V2X communication is open and vulnerable to attacks, posing privacy and safety risks to road users. Moreover, a high fatality rate is correlated with more vulnerable modes of transportation. Therefore, developing C-ITS solutions requires considering all road users' needs, not only vehicles. This study proposed a security approach within a C-ITS ecosystem while accommodating soft-mobility users and legacy vehicles. Security protocols were used in a C-ITS setting that enabled integration between connected ITS stations using ITS-G5 and soft-mobility users through smartphones over cellular technologies (hybrid networks). Two security protocols (MFSPV and DLAPP) were implemented using real hardware equipment (OBUs, RSUs and smartphones), and for each computing environment, an application was developed. Experiments were performed to evaluate the developed ecosystem. More specifically, computational, transmission and end-to-end latency were assessed. For the used experimental setup, MFSPV proved to be 16% to 113% more efficient than DLAPP, depending on the computational node and operation (protection or verification). Despite this, as both protocols were designed to be lightweight, the magnitude of the times involved is minimal, in the order of tenths of milliseconds. Moreover, both presented a low impact on local computing time compared to situations where security was not used. As for network latency, experimental measurements have shown that DLAPP is slightly more efficient as it increases G5 and cellular network latency by 6.4%, whereas MFSPV provides an 8.8% increase. Furthermore, the G5 network, on average, has 63.6% lower latency times when compared to the cellular network. Regarding the end-to-end assessment, the most time-consuming E2E communication flows were seen in the hybrid network communication flows, which is expected since messages travel via both G5 and cellular networks. In particular, the highest E2E time was 46.75 ms. Conversely, the E2E times achieved by G5-exclusive communication flows were the lowest. On average, the extension for hybrid communication imposes an extra 28.29 ms of E2E time. Concerning security, the additional E2E time imposed by using security in hybrid communications was \(\sim\)11% in both protocols. In general, the DLAPP and MFSPV protocols imposed similar additional E2E times. Therefore, choosing one over the other in terms of efficiency is not straightforward. The choice should depend on the specific priorities of the application. As a final remark, the suitability of the presented approach depends on the specific nature of the ITS applications it will incorporate. That is, different ITS use cases have distinct maximum latency demands. The most stringent ones, such as emergency services, require a 50 ms latency, and most others allow up to 100 ms [49; 50]. This study's median E2E values do not surpass \(\sim\)47 ms, aligning well with the requirements of most use cases. In a worst-case analysis, the E2E time reached around 190 ms. However, it represents an unusual scenario. Therefore, outliers were isolated using the IQR method. In this scenario, the worst-case E2E latency remained at 86 ms, 14% below the maximum latency of 100 ms. Thus, one may conclude that the obtained results align well with the requirements for many use cases. Finally, the architecture proposed for implementation in the Siemens RSU and Unex OBU presents versatile applicability to other commercial equipment. This implementation recommendation serves as a practical approach as it avoids the need for protocol stack and software modifications across diverse equipment from various manufacturers. For future work, the CA should be developed to allow evaluation of the whole prototype. A security analysis should be performed, including a risk assessment of our implementation using appropriate tools. In addition, experiments should be carried out with more OBUs and RSUs from different manufacturers.