Abstract: A method and apparatus for “ring fencing” an operating system, with any apps or programs running on that operating system, in such a way that all operations of the apps, programs, and operating system use real resources only with the permission or through the intermediation of a resource allocator programmed to allow only secure access to a known finite set of resources. The list of kinds of resources is short, and the list of resources is under the explicit control of the resource allocator, which is programmed with Communicating Sequential Processes techniques so as to give formally verifiable assurance of predefined security requirements. At the same time, the full features of the operating system are supported with minimal changes in its kernel and no changes in app or program binary code that accesses real resources via calls or bytecode that drives dynamic system libraries or a runtime environment.