This paper describes a technique that combines algebraic specifications and monads to build derivative verification condition generators (VCGs) by extending a base VCG. Extensions are compositional and can be stacked while the base VCG is left unchanged. The technique can be used to build a set of weaker VCGs, which are useful to support light weight verification. Moreover, it enables us to add an ability to generate validation traces. The paper explains the technique through an example that extends a simple language L0 with new constructs to handle exceptions. To deal with exceptions, not only that the logic of L0 has to be extended with new rules, its structure also needs to be changed. We show that using our technique the extension can be implemented in a simple and compositional way, without any change to the underlying logic. 1

Abstract — It has long been held that information flow security models should be organized with respect to a theory of information, but typically they are not. The appeal of a information-theoretic foundation for information flow security seems natural, compelling and, indeed, almost tautological. This article illustrates how channel theory—a theory of information based in logic—can provide a basis for noninterference style security models. The evidence presented here suggests that channel theory is a useful organizing principle for information flow security. 1.