A set of access privileges to protected resources. Where many processes coexist, each process having differing access permission to a number of protected resources via some form of key, it may be convenient to group together a set of such keys in order to provide a single process with access to the resources that it requires. Access control can then be manipulated independent of the processes concerned. The protection domain is either the set of keys, or equivalently, the set of resources to which the keys give access.