Lean is a theorem prover, also called a proof assistant: software that checks mathematical and logical steps with machine precision. Instead of trusting a proof by hand, Lean verifies that each inference follows from the previous ones under a small, trusted kernel. This makes it valuable anywhere correctness matters, especially in cyber security.
Security teams use Lean and similar tools to validate cryptographic algorithms, protocol specifications, access-control logic, and safety-critical code. In attacks, weak or unverified assumptions can break a system even when the implementation looks sound. In defenses, formal proofs help prove that a parser, verifier, or protocol cannot take certain bad paths. For AI-generated security analysis, Lean provides a separate verifier: a model can suggest a proof or fix, but Lean confirms whether the reasoning actually holds.