Quantum Safety is here: Signal adds SPQR to deliver quantum‑safe messaging

Signal is rolling out what they call "the Triple Ratchet", adding a Sparse Post‑Quantum Ratchet (SPQR) to their past, "Double Ratchet" ((Diffie-Hellman + symmetric-key ratcheting) Signal Protocol. ML‑KEM (Kyber) secrets are mixed with the existing Double Ratchet to create hybrid keys. Forward secrecy and post‑compromise security are thus preserved and made quantum‑safe.

The company's achievement in creating post-quantum safety have been extensively featured and lauded in the media, as an impactful step forward.

Big post‑quantum payloads are sliced into small, erasure‑coded chunks attached to messages. A parallelized ML‑KEM “Braid” sends a tiny seed+hash first, then overlaps public‑key and ciphertext chunks to minimize bandwidth.

The app experience stays the same, and sessions migrate automatically over time. SPQR supports safe initial downgrades for out‑of‑date peers and will later be enforced for all sessions.

The design and Rust implementation are formally verified with ProVerif and F*, and proofs run in CI. Work was developed with PQShield, AIST and NYU and follows PQXDH’s 2023 handshake upgrade.

Sources: Signal · Signal · Ars Technica · PqShield

B.S.