Soundness of formal encryption in the presence of active adversaries

We present a general method to prove security properties of cryptographic protocols against active adversaries, when the messages exchanged by the honest parties are arbitrary expressions built using encryption and concatenation operations. The method allows to express security properties and carry out proofs using a simple logic based language, where messages are represented by syntactic expressions, and does not require dealing with probability distributions or asymptotic notation explicitly. Still, we show that the method is sound, meaning that logic statements can be naturally interpreted in the computational setting in such a way that if a statement holds true for any abstract (symbolic) execution of the protocol in the presence of a Dolev-Yao adversary, then its computational interpretation is also correct in the standard computational model where the adversary is an arbitrary probabilistic polynomial time program. This is the first paper providing a simple framework for translating security proofs from the logic setting to the standard computational setting for the case of powerful active adversaries that have total control of the communication network.