Group Key Establishment in a Quantum-Future Scenario*

Abstract: Group key establishment protocols (GAKE) are cryptographic constructions designed for setting up secure sessions. Once a GAKE execution is finalized, the group of involved participants share a high-entropy secret that can be used as common key for different cryptographic tools. However, it is important to understand that the keys stablished today may no longer be secure in the future. In this work, we model a GAKE scenario in which adversaries may use quantum algorithms in their attacks, yet only once the actual execution of the protocol has concluded. This captures a situation in which keys are to be established in the present, while security guarantees must still be provided in the future when quantum resources may be accessible to a potential adversary. Besides giving a formal security model, we put forward a concrete design which uses password authentication and builds upon efficient and reasonably well understood primitives: a message authentication code and a post-quantum key encapsulation mechanism. The hybrid structure dodges potential efficiency downsides, like large signatures, of some “true” post-quantum authentication techniques, making our protocol a potentially interesting fit for current applications with long-term security needs. [paper published in the Journal Informatica, Vol 31, 4, pp. 751-768, 2020. ]

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