Abstract
Various formulations of non-interference have been proposed to try to characterise the absence of information flows in system or network. There is still no consensus in the information security community as to which of these accurately captures our intuition of the notion of secrecy.
We argue that non-interference is closely related to the characterisation of process equivalence. What constitutes process equivalence is itself a fundamental question in computer science with several distinct definitions proposed in the literature. We illustrate how several of the definitions of non-interference mirror notions of process equivalence. Casting these security concepts in a process algebraic framework clarifies, for example, the role of non-determinism and allows results to be carried over regarding composition and the completeness of unwinding rules. We also discuss some natural generalisations of the approach.
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