On declassification and the non-disclosure policy, with A. Matos, accepted for publication in the J. of Computer Security.

We address the issue of declassification in a language-based security approach. We introduce, in a Core ML-like language with concurrent threads, a declassification mechanism that takes the form of a local flow policy declaration. The computation in the scope of such a declaration is allowed to implement information flow according to the local policy. This dynamic view of information flow policies is supported by a concrete presentation of the security lattice, where the confidentiality levels are sets of principals, similar to access control lists. To take into account declassification, and more generally dynamic flow policies, we introduce a generalization of non-interference, that we call the non-disclosure policy, and we design a type and effect system for our language that enforces this policy. Besides dealing with declassification, our type system improves over previous systems for checking information flow in two directions: first, we show that the typing of terminations leaks can be largely improved, by particularizing the case where the alternatives in a conditional branching both terminate. Moreover, we also provide a quite precise way of approximating the confidentiality level of an expression, that ignores the level of values used for side-effects only.