ZeroMT: Multi-transfer Protocol for Enabling Privacy in Off-Chain Payments

The privacy problem in public blockchains is a well-know challenge. Despite the robustness and decentralisation properties of the blockchain, transaction information remains visible to everyone in the network. Several proposals aim at solving these issues with advanced cryptographic techniques, such as zero-knowledge proofs, which turned out to be the best candidates. However, previous works are not aimed at off-chain sessions, which often end with a transfer that involves multiple payees, coming from a single payer or from more than one. With this paper, we introduce ZeroMT, a protocol for multiple confidential balance transfers that occur in a single transaction. With this novel approach, parties of an off-chain session gain privacy for their balances and transfer amounts. In addition, all transfers occur within a single transaction that benefits scalability, reducing the number of transactions to be validated in the main-chain. We provide the generic construction of a confidential multi-transfer transaction that can be assembled off-chain and verified by smart contract platforms. As a part of our protocol, we design the multi-transfer proof system by combining the aggregate version of Bulletproofs and an extended Sigma-Protocol to n ciphertexts, generalising the Zether-SBullets protocol.