# Cybersecurity and Safety Co-Engineering of Cyberphysical Systems--A Comprehensive Survey
## 4 Discussion  
The main findings from the analysis of the literature reviewed in the previous section are the following:  
* A total of sixty eight methods have been reviewed. These span a time period of approximately 20 years, with most having been proposed after 2013, and with a steady increase in the past 5 years. This is an indication of the timeliness of the subject, which can be attributed to the increased proliferation of cyber physical systems and the integration of Information Technology with Operational Technology. * The number of integrated methods (37) is slightly larger than that of unified ones (31). According to Reference [62], approaches that attempt to unify safety and security analysis techniques reduce the developer's understanding of the system being analyzed and prevent a thorough analysis of either property; this leads to an incomplete analysis with subsequent safety and security risks going unobserved. On the other hand, integrated methods extract more rigorous results and facilitate the identification of potential conflicts. * Model-based methods prevail (52 out of 68). Of these, 18 methods employ formal models, 23 methods employ graphical models, and 11 methods employ both formal and graphical models. Model-based approaches are more practical for modeling a system's components and functionalities for existing and operational systems, by virtue of their qualitative and quantitative capabilities [4]. They are generally able to scale up to complex systems and represent different aspects related to safety and security with different viewpoints and levels of detail. On the other hand, such approaches require the analyst to have a thorough knowledge of the system; engaging all stakeholders in the process my facilitate the fulfillment of this requirement. * Less than half of the reviewed methods (20) are informed by safety and security standards. Cyberphysical systems often operate in domains and environments highly regulated by safety and security standards. Therefore, they must be engineered to conform to these standards. It follows that safety-security co-engineering methods needs to be informed by standards. This need is more often than not satisfied if the method has been designed for use in a specific application domain. Including a validation phase in the workflow of the method, in which conformance to relevant standards is performed, is a viable alternative that may lead to the development of generic methods informed by standards. A related issue, discussed in the next section, is the need for integrated safety-security standards in several application domains. * Most (45) of the reviewed methods have been used to analyze general CPS architectures and industrial control systems in various application domains, with the transportation domain prevailing. However, the applicability of the generic methods to different application domains is usually not demonstrated. Developing a method applicable to a broad spectrum of domains and at the same time ensuring compliance with relevant standards appears as a very challenging task. * The vast majority of the reviewed methods (66) follow -at least partially- a qualitative approach; only two methods are fully quantitative. This is not surprising, because even though quantitative approaches prevail for safety engineering, the opposite is true for security engineering, where quantitative approaches are very rarely used, as they require the existence of a formal model describing the system under study. Attempting to analyze security, particularly security risk, has been shown quantitatively to be either infeasible or inadvisable in most real-world situations. Hence, a reasonable compromise is to opt for a combination of quantitative and qualitative approaches for safety-security co-engineering. * The number of methods whose goal is to ensure both safety and security (32) is slightly larger than the number of those aiming to ensure safety (30), whilst only six methods have as their primary goal to ensure security.