Deciding Secrecy of Security Protocols for an Unbounded Number of Sessions: The Case of Depth-Bounded Processes

We introduce a new class of security protocols with an unbounded number of sessions and unlimited fresh data for which the problem of secrecy is decidable. The only constraint we place on the class is a notion of depthboundedness. Precisely we prove that, restricted to messages of up to a given size, secrecy is decidable for all depthbounded processes. This decidable fragment of security protocols captures many real-world symmetric key protocols, including Needham-Schroeder Symmetric Key, Otway-Rees, and Yahalom.