Microsoft Research open-sources verified Rust cryptography for SymCrypt using Lean and Aeneas

Microsoft Research reports a new method for formally verifying cryptographic code written in Rust within SymCrypt, Microsoft’s cross-product cryptographic library used in Windows and Azure. The approach uses the Lean proof framework and the Aeneas toolchain to produce machine-checked proofs that the Rust implementations correctly implement standard algorithms.

The team states that initial verified algorithms include SHA-3 and ML-KEM, and that these verified components are already included in Windows Insider builds. They note that the same Rust, Lean, and Aeneas-based workflow is being extended to additional algorithms such as AES-GCM, FrodoKEM, and ML-DSA for future Windows and Linux releases.

The methodology emphasizes keeping formal specifications close to public standards, enabling line-by-line comparison between the standard text and the Lean model. The team also reports using executable specifications to run against official test vectors to catch transcription errors or misunderstandings of the standard.

To scale verification effort, the team introduces agents that assist in writing proofs and intermediate properties, while compilation, code extraction, and proof verification remain deterministic processes. The open-sourced SymCrypt branch includes both the Rust implementations and the associated formal specifications and proofs.