  Safety and security are the two most important properties of industrial
control systems (ICS), and their integration is necessary to ensure that safety
goals do not undermine security goals and vice versa. Sometimes, safety and
security co-engineering leads to conflicting requirements or violations capable
of impacting the normal behavior of the system. Identification, analysis, and
resolution of conflicts arising from safety and security co-engineering is a
major challenge, an under-researched area in safety-critical systems(ICS). This
paper presents an STPA-SafeSec-CDCL approach that addresses the challenge. Our
proposed methodology combines the STPA-SafeSec approach for safety and security
analysis and the Conflict-Driven Clause Learning (CDCL) approach for the
identification, analysis, and resolution of conflicts where conflicting
constraints are encoded in satisfiability (SAT) problems. We apply our
framework to the Tennessee Eastman Plant process model, a chemical process
model developed specifically for the study of industrial control processes, to
demonstrate how to use the proposed method. Our methodology goes beyond the
requirement analysis phase and can be applied to the early stages of system
design and development to increase system reliability, robustness, and
resilience.
