Byzantine Fault Tolerance For Distributed Ledgers Revisited

Abstract

The problem of Byzantine Fault Tolerance (BFT) has received a lot of attention in the last 30 years. Due to the popularity of Proof of Stake (PoS) blockchains in recent years, several BFT protocols have been deployed in the large scale of Internet environment. We analyze several popular BFT protocols such as Capser FFG/CBC-FBC for Ethereum 2.0 and GRANDPA for Polkadot. Our analysis shows that the security models for these BFT protocols are slightly different from the models commonly accepted in the academic literature. For example, we show that, if the adversary has a full control of the message delivery order in the underlying network, then none of the BFT protocols for Ethereum blockchain 2.0 and Polkadot blockchain could achieve liveness even in a synchronized network. Though it is not clear whether a practical adversary could actually control and re-order the underlying message delivery system (at Internet scale) to mount these attacks, it raises an interesting question on security model gaps between academic BFT protocols and deployed BFT protocols in the Internet scale. With these analysis, this article proposes a Casper CBC-FBC style binary BFT protocol and shows its security in the traditional academic security model with complete asynchronous networks. For partial synchronous networks, we propose a multi-value BFT protocol BDLS based on the seminal DLS protocol and show that it is one of the most efficient practical BFT protocols at large scale networks in the traditional academic BFT security model. The implementation of BDLS is available at https://github.com/yonggewang/bdls . Finally, we propose a multi-value BFT protocol XP for complete asynchronous networks and show its security in the traditional academic BFT security model.