The design of usable yet secure systems raises crucial questions when it comes to balancing properly security and usability. Finding the right tradeoff between these two quality attributes is not an easy endeavor. In this paper, we introduce an original design model based on a novel usability inspection method. This new method, named Security Usability Symmetry (SUS), exploits automata machines theory and introduces the concept of an advanced Multifunction Teller Machine (MTM). We demonstrate, via case study, how to use this model during the design of secure, usable interactive systems.