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BLS Multi-Signatures

Certificates (QCs and TCs) can be naively implemented as a vector of ECDSA signatures on the secp256k1 curve. These certificates are explicit and easy to construct and verify. However, the size of the certificate is linear with the number of signers. It poses a limit to scaling because the certificate is included in almost every consensus message, except vote message. Pairing-based BLS signature on the BLS12-381 curve helps with solving the scaling issue. The signatures can be incrementally aggregated into one signature. Verifying the single valid aggregated signature provides proof that the stakes associated with the public keys have all signed on the message. BLS signature is much slower than ECDSA signature. So for performance reasons, Monad’s implementation of MonadBFT adopts a blended signature scheme where BLS signatures are only used on aggregatable message types (votes and timeouts). Message integrity and authenticity is still provided by ECDSA signatures.