This research was supported in part by the Defense Advanced Research Project Agency under the Scalable Computing Infrastructure (SCOPE) Project, TNT, Contract No. DABT63-95-C-0095, Security Infrastructure for Large Distributed systems (SILDS) Project, Contract No. DABT63-94-C-0034, and by a grant from Xerox Corporation. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of U.S. Army Intelligence Center and Fort Huachuca Directorate of Contracting, the Defense Advanced Research Project Agency, the U.S. Government, or Xerox Corporation.