Simulation based security in the applied pi calculus

We present a symbolic framework for refinement and composition of security protocols. The framework uses the notion of ideal functionalities. These are abstract systems which are secure by construction and which can be combined into larger systems. They can be separately refined in order to obtain concrete protocols implementing them. Our work builds on ideas from the ``trusted party paradigm'' used in computational cryptography models. The underlying language we use is the applied pi calculus which is a general language for specifying security protocols. In our framework we can express the different standard flavours of simulation-based security which happen to all coincide. We illustrate our framework on an authentication functionality which can be realized using the Needham-Schroeder-Lowe protocol. For this we need to define an ideal functionality for asymmetric encryption and its realization. We show a joint state result for this functionality which allows composition (even though the same key material is reused) using a tagging mechanism.