1. Department of Informatics, University of Bergen, N-5020 Bergen, Norway.
2. Department of Computer Science, University of California, Santa Barbara, CA 93106-5110 USA.
3. CISPA Helmholtz Center for Information Security, Saarland Informatics Campus, 66123 Saarbrücken, Germany.
4. Institute of Informatics, University of Warsaw, 02-097 Warsaw, Poland.
5. Institute of Mathematical Sciences, Chennai 600 113, India, and Department of Informatics, University of Bergen, N-5020 Bergen, Norway.