Type systems for information flow control: the question of granularity.

Information flow control is central to computer security. The objective of information flow control is to prevent unauthorized flows of secret information to the public outputs of a computation. This task is often accomplished using type systems that rely on modal operators to label and track information and, hence, this style of enforcing information flow control is deeply ingrained in logic. One key choice in designing a type system for information flow control, or dependence analysis in general, is the granularity at which dependencies are tracked. This article considers two extreme design points in this vast design space and examines their relative expressiveness.