Formalizing Soundness Proofs of Linear PCP SNARKs

Succinct Non-interactive Arguments of Knowledge (SNARKs) have seen interest and development from the cryptographic community over recent years, and there are now constructions with very small proof size designed to work well in practice. A SNARK protocol can only be widely accepted as secure, however, if a rigorous proof of its security properties has been vetted by the community. Even then, it is sometimes the case that these security proofs are flawed, and it is then necessary for further research to identify these flaws and correct the record [39, 58]. To increase the rigor of these proofs, we create a formal framework in the Lean theorem prover for representing a widespread subclass of SNARKs based on linear PCPs. We then describe a decision procedure for checking the soundness of SNARKs in this class. We program this procedure and use it to formalize the soundness proof of several different SNARK constructions, including the well-known Groth'16.