Signal has announced the rollout of Sparse Post-Quantum Ratchet (SPQR), a new cryptographic upgrade designed to safeguard communications against future quantum computing threats.
The system enhances Signal’s already-robust end-to-end encryption with additional security layers that ensure conversations remain protected even in the event of key compromise.
SPQR works by continuously refreshing encryption keys while discarding old ones, guaranteeing both forward secrecy and post-compromise security. Unlike traditional elliptic-curve Diffie-Hellman methods, SPQR uses post-quantum Key-Encapsulation Mechanisms (ML-KEM), with efficient chunking and erasure coding to handle large key sizes without increasing bandwidth usage.
Signal has already been deploying CRYSTALS-Kyber, a post-quantum KEM, since 2023. SPQR now builds on top of Signal’s existing Double Ratchet protocol, forming what the company calls a “Triple Ratchet.” When a message is sent, both the Double Ratchet and SPQR generate encryption keys. These are then combined through a Key Derivation Function to produce a secure “mixed key,” delivering hybrid security that’s resilient against classical and quantum attacks.
Developed in collaboration with PQShield, Japan’s AIST, and New York University, the design is based on recent academic research presented at USENIX 2025 and Eurocrypt 2025. It has also undergone formal verification using ProVerif and robustness testing with the hax tool. Signal says it will apply continuous verification to all future builds.
The SPQR upgrade will roll out gradually, requiring users only to keep their Signal apps updated. Until it’s universally deployed, conversations between SPQR-enabled and non-enabled clients will default to older security models. Once the rollout is complete, SPQR will be enforced across all sessions.
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With an estimated 100 million monthly active users, Signal’s adoption of post-quantum cryptography marks one of the most significant steps yet in preparing consumer messaging apps for the era of quantum computing.
