%O Report 
%F PI-1930
%A Cassez, F.
%A Dubreil, J.
%A Marchand, H.
%T Dynamic Observers for the Synthesis of Opaque Systems
%N 1930
%I IRISA
%X In this paper, we address the problem of synthesizing opaque systems. A secret predicate S over the runs of a system G is opaque to an external user having partial observability over G, if s/he can never infer from the observation of a run of G that the run belongs to S. We first investigate the case of static partial observability where the set of events the user can observe is fixed a priori. In this context, we show that checking whether a system is opaque is PSPACE-complete, which implies that computing an optimal static observer ensuring opacity is also a PSPACE-complete problem. Next, we introduce dynamic partial observability where the set of events the user can observe changes over time. We show how to check that a system is opaque w.r.t. to a dynamic observer and also address the corresponding synthesis problem: given a system G and secret states S, compute the set of dynamic observers under which S is opaque. Our main result is that the set of such observers can be finitely represented and can be computed in EXPTIME
%U http://hal.inria.fr/docs/00/39/64/42/PDF/PI-1930.pdf
%8 May
%D 2009