Formal Verification of a Post-Quantum Signal Protocol with Tamarin
Résumé
The Signal protocol is used by billions of people for instant messaging in applications such as Facebook Messenger, Google Messages, Signal,
Skype, and WhatsApp. However, advances in quantum computing threaten the
security of the cornerstone of this protocol: the Diffie-Hellman key exchange.
There actually are resistant alternatives, called post-quantum secure, but replacing the Diffie-Hellman key exchange with these new primitives requires a deep
revision of the associated security proof. While the security of the current Signal
protocol has been extensively studied with hand-written proofs and computerverified symbolic analyses, its quantum-resistant variants lack symbolic security
analyses.
In this work, we present the first symbolic security model for post-quantum variants of the Signal protocol. Our model focuses on the core state machines of the
two main sub-protocols of Signal: the X3DH handshake, and the so-called double ratchet protocol. Then we show, with an automated proof using the Tamarin
prover, that instantiated with the Hashimoto-Katsumata-Kwiatkowski-Prest postquantum Signal’s handshake from PKC’21, and the Alwen-Coretti-Dodis KEMbased double ratchet from EUROCRYPT’19, the resulting post-quantum Signal
protocol has equivalent security properties to its current classical counterpart.
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