The Distributed Secure System project being undertaken by the Royal Signals and Radar Establishment aims to provide distributed general-purpose computing systems capable of enforcing a variety of multi-level security policies. The project relies on the concept of synthesizing secure systems from standard untrusted components. These untrusted components are interfaced via small trustworthy security mechanisms to one or more local or wide area networks. The components are linked together in such a way that the total system is not only demonstrably secure, but is also very flexible and cost effective.