An updated threat model for security ceremonies

Since Needham and Schroeder introduced the idea of an active attacker, a lot of research has been made in the protocol design and analysis area in order to verify the protocols' claims against this type of attacker. Nowadays, the Dolev-Yao threat model is the most widely accepted attacker model in the analysis of security protocols. Consequently, there are several security protocols considered secure against an attacker under Dolev-Yao's assumptions. With the introduction of the concept of ceremonies, which extends protocol design and analysis to include human peers, we can potentially find and solve security flaws that were previously not detectable. In this paper, we discuss that even though Dolev-Yao's threat model can represent the most powerful attacker possible in a ceremony, the attacker in this model is not realistic in certain scenarios, especially those related to the human peers. We propose a dynamic threat model that can be adjusted according to each ceremony, and consequently adapt the model and the ceremony analysis to realistic scenarios without degrading security and improving usability.