Program equivalence is the fulcrum for reasoning about and proving properties
of programs. For noninterference, for example, program equivalence up to the
secrecy level of an observer is shown. A powerful enabler for such proofs are
logical relations. Logical relations only recently were adopted for session
types -- but exclusively for terminating languages. This paper scales logical
relations to general recursive session types. It develops a logical relation
for progress-sensitive noninterference (PSNI) for intuitionistic linear logic
session types (ILLST), tackling the challenges non-termination and concurrency
pose, and shows that logical equivalence is sound and complete with regard to
closure of weak bisimilarity under parallel composition, using a
biorthogonality argument. A distinguishing feature of the logical relation is
its stratification with an observation index (as opposed to a step or unfolding
index), a crucial shift to make the logical relation closed under parallel
composition in a concurrent setting. To demonstrate practicality of the logical
relation, the paper develops an information flow control (IFC) refinement type
system for ILLST, with support of secrecy-polymorphic processes, and shows that
well-typed programs are self-related by the logical relation and thus enjoy
PSNI. The refinement type system has been implemented in a type checker,
featuring local security theories to support secrecy-polymorphic processes.Comment: arXiv admin note: text overlap with arXiv:2208.1374