Signal Introduces Hybrid Post-Quantum Ratchet to Strengthen Security

Signal has announced a major cryptographic upgrade, introducing the Sparse Post Quantum Ratchet (SPQR) which combines its existing Double Ratchet with SPQR to form a Triple Ratchet system. According to Signal, the hybrid protocol aims to protect user communications not only against current threats but also future quantum computer attacks, while preserving forward secrecy and post-compromise security.

The rollout is designed to be seamless for users, upgrading conversations in the background and using intelligent downgrade to allow devices with different protocol versions to continue communicating securely. SPQR builds on the previously introduced PQXDH (Post-Quantum Extended Diffie-Hellman) to extend quantum resistance across entire conversation lifecycles, with the development supported by academic collaboration with PQShield, AIST, and NYU and peer-reviewed papers at Eurocrypt and USENIX Security.

The project emphasises formal verification, leveraging ProVerif and F* to produce machine-verified proofs that security properties hold with updates, reflecting Signal’s proactive posture toward a post-quantum world. October 3, 2025