Nondeterministic Admissible Interference

In this article we address the issue of con dentiality of information in the context of downgrading systems i.e. systems admitting information ow between secrecy levels only through a downgrader. Inspired by the intuition underlying the usual de nition of admissible information ow, we propose an analogue based on trace equivalence as developed in the context of concurrency theory and on a modi cation of the usual de nition of purge function. We also provide unwinding conditions to guarantee a consistent and complete proof method in terms of communicating transition systems. We take advantage of this framework to investigate its compositionality issues w.r.t. the main operators over communicating transition systems. We conclude the article with a short presentation of this work's most promising aspects in the perspective of future developments.