# Cybersecurity and Safety Co-Engineering of Cyberphysical Systems--A Comprehensive Survey
## 4 Discussion
The appropriateness of each of these approaches depends largely on the system's safety/security criticality. When the system under study is safety critical, a method whose goal is to ensure that security will not adversely influence safety is appropriate; the opposite is true when the system is security critical. But if the intention is to also have a secure system beyondthe safety relevant security issues, and a safe system beyond the security relevant issues, then neither of these approaches is appropriate. In systems where both safety and security are equally important, an approach aiming to ensure both safety and security would be more appropriate. * The number of reviewed methods that are applied to both the requirements elicitation and risk analysis phases of the system lifecycle (26) is almost equal to that of methods applied to the risk analysis phase (25), and only slightly exceeds that of methods applied to the requirements elicitation phase (17); only fifteen methods are frameworks, hence applicable to any phase of the lifecycle. This nearly uniform distribution reflects the emphasis given into co-engineering safety and security as early as possible in the development lifecycle, while allowing for revisiting the results of the analysis when the system under study has been developed or is even operational. * The application of most of the reviewed methods involves safety and security experts; only few methods require the engagement of developers, designers and systems users. It is important to note that stakeholders, particularly designers and users, may engage with the analysis in two distinct but complementary ways: they provide input to the analysis in the form of domain expert knowledge, and they are the targets of the process of communicating the results; both are equally important. Acknowledging the fact that complex issues such as those of safety and security cannot be effectively analysed by the corresponding experts, it follows that successful methods will seek to involve engaged stakeholders. * Scalability issues have been discussed in the vast majority (61) of the papers proposing methods, in 24 of which these issues have only briefly been considered. Thirty seven methods are scalable, whereas 7 do not scale well. It should be noted that scalability refers to both the ability of the method to handle complex systems and to the level of abstraction at which the system under study is represented. The two are correlated, as high level abstraction allows for more complex systems to be analyzed. The challenge, therefore, is to develop methods that can strike an appropriate balance between those two aspects of scalability, so that the analysis results in an appropriate and practically useful level of detail. * The majority (55) of the reviewed methods provide mechanisms to stimulate creativity among the analysts and other relevant stakeholders. As many methods rely, at least to some extent, on scenario development, creativity is an important characteristic. This is even more so when the application of the method calls for a multi-disciplinary, multi-stakeholder team. * Less than half (28) of the reviewed methods include techniques to communicate their results to the relevant stakeholders. Another 28 methods only briefly address the issue, whilst 10 methods do not address it at all. As already implied above, this characteristic is intertwined with the involvement of stakeholders attribute. * All methods are process-based; the structure and the steps of the process do vary, however. As pointed out in the sequel, developing a methodology to encompass different process structures is still a challenge. * The majority (49) of the reviewed methods do not address the conflict resolution issue. Sixteen methods do address it, and a further 3 address it only partially. The implications of this central issue is elaborated upon in the sequel. * The majority (41) of the reviewed methods are not supported by any software tool or toolkit.