Using Throughput-Centric Byzantine Broadcast to Tolerate Malicious Majority in Blockchains

Fault tolerance of a blockchain is often characterized by the fraction f of “adversarial power” that it can tolerate in the system. Despite the fast progress in blockchain designs in recent years, existing blockchain systems can still only tolerate f below 0.5. Can practically usable blockchains tolerate a malicious majority, i.e., f above 0.5? This work presents a positive answer to this question. We first note that the well-known impossibility of byzantine consensus for f above 0.5 does not carry over to blockchains. To tolerate f above 0.5, we use byzantine broadcast, instead of byzantine consensus, as the core of the blockchain. A major obstacle in doing so, however, is that the resulting blockchain may have extremely low throughput. To overcome this central technical challenge, we propose a novel byzantine broadcast protocol OverlayBB, that can tolerate f above 0.5 while achieving good throughput. Using OverlayBB as the core, we present the design, implementation, and evaluation of a novel Proof-of-Stake blockchain called BCube. BCube can tolerate a malicious majority, while achieving practically usable transaction throughput and confirmation latency in our experiments with 10000 nodes and under f=0.7. To our knowledge, BCube is the first blockchain that can achieve such properties.