PROJECTS: This research was partially sup- ported by the National Science Foundation under Grants Nos. 0311808, 0433540, 0448452, 0627511, and CCF-0424422, by the Air Force Office of Scientific Research under MURI Grant No. 22178970-4170, by the Army Research Office under the Cyber-TA Research Grant No. W911NF-06-1-0316, and by CyLab at Carnegie Mellon under Grant No.DAAD19-02-1-0389 from the Army Research Office. During the preparation of this manuscript, Juan Caballero was partially funded by the European Union through Grants FP7- ICT No. 256980 and FP7-PEOPLE-COFUND No. 229599 and by the Spanish Government through a Juan de la Cierva Fellowship. PARTNER: IMDEA CITATIONS: 4