Secure Composition of Untrusted Code: Box π, Wrappers, and Causality Types

Software systems are becoming heterogeneous: instead of a small number of large programs from well-established sources, a user’s desktop may now consist of many smaller components that interact in intricate ways. Some components will be downloaded from the network from sources that are only partially trusted. A user would like to know that a number of security properties hold, e.g. that personal data is not leaked to the net, but it is typically infeasible to verify that such components are well-behaved. Instead, they must be executed in a secure environment that provides fine-grain control of the allowable interactions between them, and between components and other system resources. In this paper, we consider the problem of assembling concurrent software systems from untrusted or partially trusted off-the-shelf components, using wrapper programs to encapsulate components and enforce security policies. We introduce a model programming language, the box-π calculus, that supports composition of software components and the enforcement of information flow security policies. Several example wrappers are expressed using the calculus; we explore the delicate security properties they guarantee. We present a novel causal type system that statically captures the allowed flows between wrapped possibly-badly-typed components; we use it to prove that an example ordered pipeline wrapper enforces a causal flow property. Correspondence Address Peter Sewell University of Cambridge Computer Laboratory JJ Thomson Avenue Cambridge CB3 0FD UK [email protected] Tel: +44 1223 33 46 24, fax: +44 1223 33 46 78.