Overview

• Signal adds a third ratchet layer to its protocol, extending Double Ratchet to a Triple Ratchet to counter harvest-now-decrypt-later threats.
• The new component relies on ML-KEM 768, which Signal says has been reviewed and standardized by NIST.
• To manage large post-quantum keys, SPQR distributes fragments across multiple messages using erasure codes so any required subset enables a key update.
• For one-sided chat patterns, Signal introduces an incremental variant called ML-KEM Braid to accelerate key rollover.
• A phased rollout keeps chats working with older clients and permits downgrades only at conversation start, with users simply installing updates as peer review and Rust-to-F* formal verification proceed.