Interaction and Causality in Digital Signature Exchange Protocols

Causal reasoning is a powerful tool in analysing security protocols, as seen in the popularity of the strand space model. However, for protocols that branch, more subtle models are called for to capture the ways in which they can interact with a possibly malign environment. We study a model for security protocols encompassing causal reasoning and interaction, developing a semantics for a simple security protocol language based on concurrent games played on event structures. We show how it supports causal reasoning about a protocol for secure digital signature exchange. The semantics paves the way for the application of more sophisticated forms of concurrent game, for example including symmetry and probability, to the analysis of security protocols.