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

Signal’s post-quantum makeover is described as a major engineering achievement, adding a third quantum-safe ratchet called SPQR to the existing double ratchet while preserving forward secrecy and post‑compromise security. The update introduces ML-KEM 768, a quantum-safe key encapsulation mechanism based on CRYSTALS-Kyber, alongside PQXDH, to make the initial handshake quantum-resistant, and then couples this with a parallel SPQR ratchet to protect ephemeral keys that remain vulnerable to quantum attacks.

To manage the increased key material, the team explored options and settled on breaking the KEM into smaller chunks and using erasure codes to tolerate packet loss, allowing the new secrets to be established even when data segments are dropped. The design combines a classical DH-based ratchet with the new quantum-safe ratchet, mixing keys from both to produce a single encryption key, ensuring security even if one ratchet is compromised.

Outside researchers have praised the work; for example, Matt Green described the achievement as “an amazing engineering achievement” in reference to Sneaking an elephant through a tunnel designed for cats. The design was developed in collaboration with PQShield, AIST, and New York University, and discussed at the Eurocrypt 2025 conference, Usenix 25, and the NIST PQC Standardization Conference.