Categorising non-interference

Noninterference (see J.A. Goguen and J. Meseguer, 1982) is given an abstract definition in category-theoretic terms. Unwinding theorems are investigated from this starting point. The theorems assume that commands form a monoid. Thus the results do not apply to systems where some sequences of commands are syntactically invalid. The extension to categories would generalize the results to languages where not every string is a syntactically valid program. It is concluded that category theory is a powerful tool for reasoning about noninterference