Abstract
This paper uses CSP to introduce a characterisation of noninterference in terms of the deductions that may be made about high level processes by low level tests. May testing yields classic noninference, and has a concise formulation in CSP. It is preserved by a wider range of composition operators than are normally considered in the context of non-interference, and thus also composes under the operators traditionally studied with non-interference. The CSP characterisation of may noninterference also permits some attractive and simple Compositionality proofs. This work has benefitted from discussions with Peter Ryan, and from the careful reading and comments of the anonymous referees. The work has also received financial support from DERA.