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 using authorizations associated with at least one principal included in at least one of said digital certificates by performing least fixpoint computations using said authorizations.