Google DeepMind announced AlphaProof Code on November 10, a multi-agent formal-verification framework, and used it on a hard real target: the actual compiled code behind Signal's cryptographic protocol, which also underpins private messaging in apps such as Signal and Google Messages. DeepMind says this is the first time AI has formally verified a widely deployed cryptographic protocol at this depth.

Human experts worked with models to turn Signal's security goals into formal specifications, then Gemini-powered agents generated the proofs — over six million lines of Lean, the machine-checkable proof language — covering properties including the PQXDH handshake and the Triple Ratchet in the compiled AArch64 binary built from unmodified libsignal code. The work took four weeks, a job DeepMind says historically takes expert teams years. As a sanity check, the team reintroduced two real, previously fixed bugs into an isolated copy and the system proved the buggy version insecure in both cases. DeepMind is open-sourcing the verification artifacts.

The bounds matter: the proofs cover specified properties of one-to-one session code, not every part of Signal, group messaging, multi-device state or side channels, and they rest on human-reviewed specifications and assumptions. Signal's CTO Ehren Kret said the work gives the company greater confidence in its code's privacy guarantees.