We present a cut-admissible system for the modal logic S5 in a formalism that makes explicit and intensive use of deep inference. Deep inference is induced by the methods applied so far in conceptually pure systems for this logic. The system enjoys systematicity and modularity, two important properties that should be satisfied by modal systems. Furthermore, it enjoys a simple and direct design: the rules are few and the modal rules are in exact correspondence to the modal axioms.

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Abstract. Information systems support data privacy by constraining user's access to public views and thereby hiding the non-public underlying data. The privacy problem is to prove that none of the private data can be inferred from the information which is made public. We present a formal definition of the privacy problem which is based on the notion of certain answer. Then we investigate the privacy problem in the contexts of relational databases and ontology based information systems.