AHB: Post-Quantum Ad Hoc Blockchain for Private Barter (Extended Abstract)

Off-chain blockchain contracts are useful for parties who wish to keep the details of their contract's business logic private. The ad hoc installation of blockchain instances on mutually agreed-upon servers forms a reliable infrastructure for such contracts. We detail the core post-quantum algorithmic ingredients for structuring ad hoc blockchain (AHB) infrastructure and contracts. The initialization of AHB involves agreement on servers and blockchain software, such as Ethereum or the post-quantum SodsBC versions [13]. These choices, in turn, are approved by the digital signatures of the contract binding parties. A contract in AHB is executed by an MPC-based zero-knowledge proof of a hash (SHA) preimage commitment. Our efficient implementation extends zero-knowledge proofs based on MPC-in-The-Head (or MitH, in short) to the case of distributed verified signatures with a global zero-knowledge verification of knowledge of secret shared preimage. AHB guarantees that the preimage is released when a group with a predefined number of participants running the AHB agrees that the off-chain contract conditions are met. A new key aspect of AHB is the opportunity to eliminate all but the current state of the MPC history while executing the MPC, as we can verify that the result is identical to the declared SHA value of the preimage. Although AHB is a multi-round MPC, it transfers and stores less data than MitH. Furthermore, it maintains the same level of privacy, as participants are exposed solely to provably random information.