Typing access control and secure information flow in sessions

We consider a calculus for multiparty sessions with delegation, enriched with security levels for session participants and data. We propose a type system that guarantees both session safety and a form of access control. Moreover, this type system ensures secure information flow, including controlled forms of declassification. In particular, it prevents information leaks due to the specific control constructs of the calculus, such as session opening, selection, branching and delegation. We illustrate the use of our type system with a number of examples, which reveal an interesting interplay between the constraints of security type systems and those used in session types to ensure properties like communication safety and session fidelity.