Strain: A Secure Auction for Blockchains

We present Strain, a new auction protocol running on top of blockchains and guaranteeing bid confidentiality against fully-malicious parties. As our goal is efficiency and low blockchain latency, we abstain from using traditional, highly interactive MPC primitives such as secret shares. Instead for Strain, we design a new maliciouslysecure two-party comparison mechanism executed between any pair of bids in parallel. Using zero-knowledge proofs, Strain broadcasts the outcome of comparisons on the blockchain in a way such that all parties can verify each outcome. The resulting latency is constant in both the number of parties and the bid length, i.e., asymptotically optimal. It is also low in practice, requiring only a total of 4 blocks. Strain also provides typical auction security requirements like non-retractable bids against fully-malicious adversaries. Finally, it protects against adversaries aborting the auction by reversible commitments.