1. Dipartimento di Matematica e Informatica, Università di Catania, Catania, Italy. E-mail: giamp@dmi.unict.it
2. School of Electronic Engineering and Computer Science, Queen Mary University of London, London, UK. E-mail: p.curzon@qmul.ac.uk
3. Interdisciplinary Centre for Security, Reliability and Trust, University of Luxembourg, Luxembourg. E-mail: gabriele.lenzini@uni.lu