Using Formal Methods in a Retrospective Safety Case

Theimportanceofsafetyandsecurityisgrowingsteadily.Safetyisaqualityc- racteristic that traditionally has been considered to be important in embedded systems, and security is usually an essential property in business applications. There is certainly a tendency to use software-based solutions in safety-critical applications domains, which increases the importance of safety engineering te- niques. These include modelling and analysis techniques as well as appropriate processes and tools. And it is surely correct that the amount of con?dential data that require protection from unauthorized access is growing. Therefore, security is very important. On the one hand, the traditional motivations for addressing safety and security still exist, and their relevance has improved. On the other hand, safety and security requirements occur increasingly in the same system. At present, many software-based systems interact with technical equipment and they communicate, e.g., with users and other systems. Future systems will more and more interact with many other entities (technical systems, people, the en- ronment). In this situation, security problems may cause safety-related failures. It is thus necessary to address safety and security. It is furthermore required to take into account the interactions between these two properties.