1. Department of Computer Science, Institute of Information Security, ETH Zurich, Zurich, Switzerland. E-mail: basin@inf.ethz.ch
2. Schindler Elevators Ltd, Ebikon, Switzerland. E-mail: samuel.burri@ch.schindler.com
3. IBM Research – Zurich, Rüschlikon, Switzerland. E-mail: karjoth@acm.org