User-mediated authentication protocols and unforgeability in key collision

User interaction constitutes a largely unexplored field in protocol analysis, even in instances where the user takes an active role as a trusted third party, such as in the Internet of Things (IoT) device initialization protocols. Initializing the formal modeling of 3-party authentication protocols where one party is a physical user, this research introduces the 3-party possession user-mediated authentication (3-PUMA) model. The 3-PUMA model addresses active user participation in a protocol which is designed to authenticate possession of a fixed data string—such as in IoT device commissioning. Using the 3-PUMA model, we provide a computational analysis of the ISO/IEC 9798-6:2010 standard’s Mechanism 7a authentication protocol which includes a user interface and interaction as well as a device-to-device channel. Furthermore, we introduce existential unforgeability under key collision attacks (EUF-KCA) and provide a corresponding security experiment. We show that the security of ISO/IEC 9798-6:2010 Mechanism 7a relies upon EUF-KCA MAC security. Since it is unknown whether any standardized MAC algorithm achieves EUF-KCA security, this research demonstrates a potential vulnerability in the standard.