Confined Types

Sharing and transfer of object references is difficult to control in object-oriented languages. Unconstrained sharing poses serious problems for writing secure components in object-oriented languages. In this paper, we present a set of inexpensive syntactic constraints that strengthen encapsulation in object-oriented programs and facilitate the implementation of secure systems. We introduce two mechanisms: confined types to impose static scoping on dynamic object references and, for technical reasons, anonymous methods which are methods that do not reveal the identity of the current instance (this). Confined types protect objects from use by untrusted code, while anonymous methods allow standard classes to be reused from confined classes. We have implemented a verifier which performs a modular analysis of Java programs and provides a static guarantee that confinement is respected. We present security related programming examples.