Behavior-preserving opacity enforcement via transition modification

Develop an opacity-enforcement framework for partially observed discrete-event systems modeled as finite-state automata that preserves the system’s full behavior (i.e., does not remove or create behaviors) by disabling, adding, and/or replacing transitions without creating new behavior, such that an intruder with full structural knowledge cannot infer whether structural modifications were made, while still enforcing strong state-based opacity (e.g., K-SSO, SCSO, SISO, or Inf-SSO).

Background

The paper introduces an enforcement mechanism for strong state-based opacity that works by selecting controllable transitions to disable before execution, effectively restricting the system’s behavior to eliminate runs that violate opacity. While effective and efficient, this approach cannot be applied when the system must execute its full behavior, since it inherently removes behaviors.

In the concluding remarks, the authors propose extending the enforcement approach by allowing disabling, adding, and/or replacing transitions, provided that no new behavior is created. The goal is to preserve the original behavior set while obfuscating the structure so that an intruder, even with full structural knowledge, cannot tell whether modifications were made. They explicitly leave this research direction for future study.

References

The proposed opacity-enforcement mechanism, in this paper, is to restrict the original system's behavior to ensure that the system does not reveal its "secrets" to an intruder. However, it does not apply to such a scenario where the system must execute its full behavior. To overcome this drawback, it is reasonable to extend the proposed opacity-enforcement approach by disabling, adding, and/or replacing transitions without creating new behavior. Thus, an intruder still cannot learn for sure whether the structure of the original system has been modified based on his/her full knowledge of the system structure. The interesting direction is left for future study.

— Verification and Enforcement of Strong State-Based Opacity for Discrete-Event Systems  (2401.10363 - Han et al., 2024) in Section 5, Concluding remarks