Faceted Dynamic Information Flow Via Control And Data Monads

An application that fails to ensure information flow security may leak sensitive data such as passwords, credit card numbers, or medical records. News stories of such failures abound. Austin and Flanagan [2] introduce faceted values -values that present different behavior according to the privilege of the observer -as a dynamic approach to enforce information flow policies for an untyped, imperative lambda-calculus.We implement faceted values as a Haskell library, elucidating their relationship to types and monadic imperative programming. In contrast to previous work, our approach does not require modification to the language runtime. In addition to pure faceted values, our library supports faceted mutable reference cells and secure facet-aware socket-like communication. This library guarantees information flow security, independent of any vulnerabilities or bugs in application code. The library uses a control monad in the traditional way for encapsulating effects, but it also uniquely uses a second data monad to structure faceted values. To illustrate a non-trivial use of the library, we present a bi-monadic interpreter for a small language that illustrates the interplay of the control and data monads.