Opacity From Observers With a Bounded Memory

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 letter, 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.