1. A system for controlling access to electronic resources, the system comprising: a first computer system for processing requests for system resources, the first computer system comprising: a network interface for receiving digital certificates from other systems and for receiving requests to access electronic resources; a memory for storing electronic resources and one or more certificates relating thereto; and a trust management engine for processing digital certificates and requests for electronic resources, and for making access control decisions by performing least fixpoint computations using said digital certificates.