Studying interrelationships of safety and security for software assurance in cyber-physical systems: Approach based on bayesian belief networks

The paper discusses mutual relationships of safety and security properties in cyber-physical systems (CPS). Generally, safety impacts the system's environment while environment impacts security of a CPS. Very frequently, safety and security of a CPS interact with each other either synergistically or conflictingly. Therefore, a combined evaluation of safety and security that considers their interrelationships is required for proper assessment of a CPS. Bayesian Belief Networks (BBN) can be used for this evaluation where factors related to safety and security of a CPS are assumed to be randomly distributed. The result of this evaluation is an assessment that is non-deterministic in nature but gives a very good approximation of the actual extent of safety and security in a CPS. Using a case study of a SCADA system in an oil pipeline control, the authors present a BBN approach for assessing mutual impacts of security and safety violations. This approach is compared with the Non-Functional Requirements approach (NFR), used previously, which is largely qualitative in nature. This study demonstrates that the BBN approach can significantly complement other techniques for joint assessment of safety and security in CPS.