Pairings in Rank-1 Constraint Systems

International audienceBilinear pairings have been used in different cryptographic applications and demonstrated to be a key building block for a plethora of constructions. In particular, some Succinct Non-interactive ARguments of Knowledge (SNARKs) have very short proofs and very fast verification thanks to a multi-pairing computation. This succinctness makes pairing-based SNARKs suitable for proof recursion, that is proofs verifying other proofs. In this scenario one requires to express efficiently a multi-pairing computation as a SNARK arithmetic circuit. Other compelling applications such as verifying Boneh-Lynn-Shacham (BLS) signatures or Kate-Zaverucha-Goldberg (KZG) polynomial commitment opening in a SNARK fall into the same requirement. The implementation of pairings is challenging but the literature has very detailed approaches on how to reach practical and optimized implementations in different contexts and for different target environments. However, to the best of our knowledge, no previous publication has addressed the question of efficiently implementing a pairing as a SNARK arithmetic circuit. In this work, we consider efficiently implementing pairings in Rank-1 Constraint Systems (R1CS), a widely used model to express SNARK statements. We implement our techniques in the gnark open-source ecosystem and show that the arithmetic circuit depth can be almost halved compared to the previously best known pairing implementation on a Barreto-Lynn-Scott (BLS) curve of embedding degree 12, resulting in a significantly faster proving time. We also investigate and implement the case of BLS curves of embedding degree 24