Why Signal’s post-quantum makeover is an amazing engineering achievement

Signal has achieved a significant engineering feat by making its communication protocol quantum-resistant, addressing the threat posed by future quantum computers. The article details the complex integration of larger quantum-safe algorithms like ML-KEM-768 into Signal's existing "double ratchet" system, which constantly evolves encryption keys. To handle increased key sizes and maintain asynchronous messaging, developers implemented "erasure codes" and a novel "Sparse Post Quantum Ratchet" (SPQR), effectively adding a third, parallel ratchet. This ensures the protocol's forward secrecy and post-compromise security are protected against future quantum attacks, setting a high standard for post-quantum readiness in end-to-end encrypted messaging.