Strand Spaces with Choice via a Process Algebra Semantics

Roles in cryptographic protocols do not always have a linear execution, but may include choice points causing the protocol to continue along different paths. In this paper we address the problem of representing choice in the strand space model of cryptographic protocols, particularly as it is used in the Maude-NPA cryptographic protocol analysis tool. To achieve this goal, we develop and give formal semantics to a process algebra for cryptographic protocols that supports a rich taxonomy of choice primitives for composing strand spaces. In our taxonomy, deterministic and non-deterministic choices are broken down further. Non-deterministic choice can be either explicit, i.e., one of two paths is chosen, or implicit, i.e., the value of a variable is chosen non-deterministically. Likewise, deterministic choice can be either an explicit if-then-else choice, i.e., one path is chosen if a predicate is satisfied, while the other is chosen if it is not, or implicit deterministic choice, i.e., execution continues only if a certain pattern is matched. We have identified a class of choices which includes finite branching and some cases of infinite branching, which we address in this paper. We provide a bisimulation result between the expected forwards execution semantics of the new process algebra and the original symbolic backwards semantics of Maude-NPA that preserves attack reachability. We have fully integrated the process algebra syntax and its transformation into strands in Maude-NPA. We illustrate its expressive power and naturalness with various examples, and show how it can be effectively used in formal analysis. This allows users to write protocols from now on using the process syntax, which is more convenient for expressing choice than the strand space syntax, in which choice can only be specified implicitly, via two or more strands that are identical until the choice point.