Automated Analysis of Equivalence Properties for Security Protocols Using Else Branches

In this paper we present an extension of the AKISS protocol verification tool which allows to verify equivalence properties for protocols with else branches, i.e., disequality tests. While many protocols are represented as linear sequences or inputs, outputs and equality tests, the reality is often more complex. When verifying equivalence properties one needs to model precisely the error messages sent out when equality tests fail. While ignoring these branches may often be safe when studying trace properties this is not the case for equivalence properties, as for instance witnessed by an attack on the European electronic passport. One appealing feature of our approach is that our extension re-uses the saturation procedure which is at the heart of the verification procedure of AKISS as a black box, without need to modify it. As a result we obtain the first tool that is able verify equivalence properties for protocols that may use xor and else branches. We demonstrate the tool’s effectiveness on several case studies, including the AKA protocol deployed in mobile telephony.