Steve Kremer

Myrto Arapinis, Stéphanie Delaune, Steve Kremer

The design and verification of cryptographic protocols is a notoriously difficult task, even in symbolic models which take an abstract view of cryptography. This is mainly due to the fact that protocols may interact with an arbitrary attacker which yields a verification problem that has several sources of unboundedness (size of messages, number of sessions, etc. In this paper, we characterize a class of protocols for which deciding security for an unbounded number of sessions is decidable. More precisely, we present a simple transformation which maps a protocol that is secure for a bounded number of protocol sessions (a decidable problem) to a protocol that is secure for an unbounded number of sessions. The precise number of sessions that need to be considered is a function of the security property and we show that for several classical security properties a single session is sufficient. Therefore, in many cases our results yields a design strategy for security protocols: (i) design a protocol intended to be secure for a {single session}; and (ii) apply our transformation to obtain a protocol which is secure for an unbounded number of sessions.

Steve Kremer, Robert Künnemann

Security APIs, key servers and protocols that need to keep the status of transactions, require to maintain a global, non-monotonic state, e.g., in the form of a database or register. However, most existing automated verification tools do not support the analysis of such stateful security protocols - sometimes because of fundamental reasons, such as the encoding of the protocol as Horn clauses, which are inherently monotonic. A notable exception is the recent tamarin prover which allows specifying protocols as multiset rewrite (msr) rules, a formalism expressive enough to encode state. As multiset rewriting is a "low-level" specification language with no direct support for concurrent message passing, encoding protocols correctly is a difficult and error-prone process. We propose a process calculus which is a variant of the applied pi calculus with constructs for manipulation of a global state by processes running in parallel. We show that this language can be translated to msr rules whilst preserving all security properties expressible in a dedicated first-order logic for security properties. The translation has been implemented in a prototype tool which uses the tamarin prover as a backend. We apply the tool to several case studies among which a simplified fragment of PKCS\#11, the Yubikey security token, and an optimistic contract signing protocol.