Opacity From Observers With a Bounded Memory

Abstract

Opacity is an information-flow property capturing privacy from observers that are aware of a system’s dynamics. The potential for an observer with perfect recall to reason about long histories of the system poses a challenge for opacity verification. In this paper, we address this challenge by proposing a new notion of opacity over automata, called bounded memory opacity, with respect to an observer with a bounded memory. We show that verifying this weaker notion of opacity has reduced computational complexity compared to general opacity (co-NP vs. PSPACE). Furthermore, we present a corresponding verification algorithm using an encoding to the Boolean satisfiability problem (SAT). We demonstrate this approach on randomly generated automata as well as a web server load-hiding example.