Pisces: A New Zero-Knowledge Protocol for Blockchain Privacy

Applications of blockchain in banking, health care, transportation, asset and supply chain require to maintain the privacy of transactions, which can be achieved through anonymity using generic non-interactive zero-knowledge proof systems. In this work, we design and evaluate a simple zero-knowledge argument protocol for arithmetic circuit satisfiability to present verifiable encryption proof, which can offer good concrete efficiency and sublinear communication in the circuit size when combined with the regular signing process of the blockchain transactions. The proposed zero-knowledge protocol is an improved and optimized version of the lightweight sublinear protocol called Ligero (CCS 2017). The proof system requires no trusted setup, is plausibly post-quantum secure and uses only lightweight cryptography. We report on experiments for evaluating the performance of our proposed protocol. For instance, for verifying a SHA-256 preimage in zero-knowledge with 128 bits security, the communication cost can be roughly reduced to 1/4 and the proof size can be shortened to 3/4, compared with the original protocol. While the prover running time has a slight improvement, the verifier running time is 4x shorter than Ligero. This brings great advantages in practice, as the transactions conducted on a block (created by a miner in general) must be verified by the network (many nodes in general) before the block can be added to the chain.