Google DeepMind researchers have achieved a significant milestone in software security by using AlphaProof Code, a novel AI framework, to formally verify the core cryptographic protections in the Signal protocol. The AI agents wrote over three million lines of machine-checked proof in Lean to verify the code down to machine instructions, ensuring the security of billions of private conversations in apps like Signal and Google Messages. This breakthrough marks the first time AI has been used to formally verify a widely deployed cryptographic protocol, combining human expertise with AI agents to achieve the highest standard of security assurance, and demonstrates a major advancement in using AI for formal verification, a previously labor-intensive process.