Comparing the Notions of Opacity for Discrete-Event Systems

TL;DR

This paper explores various notions of opacity in discrete-event systems, demonstrating their inter-transformability, and provides improved algorithms and complexity results for verifying these opacity properties.

Contribution

It extends previous work by showing all opacity notions are transformable, and offers more efficient algorithms with better complexity bounds for their verification.

Findings

All opacity notions are mutually transformable in polynomial time.

The transformations preserve observable events and determinism.

Verification algorithms for certain opacity notions are significantly improved.

Abstract

Opacity is an information flow property characterizing whether a system reveals its secret to a passive observer. Several notions of opacity have been introduced in the literature. We study the notions of language-based opacity, current-state opacity, initial-state opacity, initial-and-final-state opacity, K-step opacity, and infinite-step opacity. Comparing the notions is a natural question that has been investigated and summarized by Wu and Lafortune, who provided transformations among current-state opacity, initial-and-final-state opacity, and language-based opacity, and, for prefix-closed languages, also between language-based opacity and initial-state opacity. We extend these results by showing that all the discussed notions of opacity are transformable to each other. Besides a deeper insight into the differences among the notions, the transformations have applications in complexity results. In particular, the transformations are computable in polynomial time and preserve the number of observable events and determinism, and hence the computational complexities of the verification of the notions coincide. We provide a complete and improved complexity picture of the verification of the discussed notions of opacity, and improve the algorithmic complexity of deciding language-based opacity, infinite-step opacity, and K-step opacity.