Serial Compositional Runtime Enforcement of Safety Timed Properties

Runtime enforcement is a mechanism that compels a (black-box) system to obey some expected properties. For that, it employs an enforcement monitor /enforcer which modifies an (untrusted) sequence of events into a sequence that complies with the property. In reality, we may have many critical (timed) properties to enforce. Furthermore, an ideal deployed system allows for system customization to meet the needs of the end-users. Thus, it is highly needed to build not a monolithic enforcer for all the properties, instead, there must be individual enforcers for each property and these individual enforcers should be composed accordingly. We investigate and provide a framework for composing enforcers of (timed) safety properties, formalized as timed automata, demonstrating that enforcement under this approach is not serially compositional in general. However, we identify and establish syntactic criteria on the automata, such that enforcers are serially compositional for any given set of safety timed automata satisfying these conditions. We show some examples of safety timed automata that satisfy those syntactic constraints, and via a prototype implementation, we evaluate the performance of our framework.