Lyra: Fast and Scalable Resilience to Reordering Attacks in Blockchains.

Reordering blockchain transactions to manipulate markets profited hackers by hundreds of millions of dollars. Because they rely on State Machine Replication (SMR), blockchains order transactions without preventing hackers from influencing the chosen order. Some order-fair consensus protocols, like Pompe [33], order transactions before agreeing on this order. They are insufficient because a hacker can leverage the lack of triangle inequality among network latencies to observe pending transactions before issuing their own. Other DAG-based protocols, like Fino [24], use commit-reveal to obfuscate transactions, but cannot prevent reordering by a Byzantine leader. In this paper, we present Lyra, a protocol that solves this problem. The key idea is the combination of a commit-reveal protocol to obfuscate transaction payloads, and a leaderless ordered consensus protocol that predicts the order of transactions. Lyra has optimal good-case latency, prevents reordering attacks, and is scalable. Finally, it outperforms the latency of Pompe by up to 2 times and its throughput by up to 7 times on a 100-node network over 3 continents.