  Mature push button tools have emerged for checking trace properties (e.g.
secrecy or authentication) of security protocols. The case of
indistinguishability-based privacy properties (e.g. ballot privacy or
anonymity) is more complex and constitutes an active research topic with
several recent propositions of techniques and tools. We explore a novel
approach based on type systems and provide a (sound) type system for proving
equivalence of protocols, for a bounded or an unbounded number of sessions. The
resulting prototype implementation has been tested on various protocols of the
literature. It provides a significant speed-up (by orders of magnitude)
compared to tools for a bounded number of sessions and complements in terms of
expressiveness other state-of-the-art tools, such as ProVerif and Tamarin:
e.g., we show that our analysis technique is the first one to handle a faithful
encoding of the Helios e-voting protocol in the context of an untrusted ballot
box.
