Ownership Downgrading for Ownership Types

Ownership types support information hiding by providing object-based encapsulation. However the static restrictions they impose on object accessibility can limit the expressiveness of ownership types. In order to deal with real applications, it is sometimes necessary to admit mechanisms for dynamically exposing otherwise encapsulated information. The need for policies and mechanisms to control such information flow, known as downgrading or declassification, has been well covered in the security literature. This paper proposes a flexible ownership type system for object-level access control. It still maintains privacy of owned data, but allows information to be dynamically exposed where appropriate through an explicit declassification operation. The key innovation is an owners-as-downgraders policy, implemented via a simple language construct, which allows an object to be made more widely accessible by downgrading its ownership to its owner’s owner.