Censorship by Procrastination Attack in Leader-Based BFT Blockchains

Consensus is a fundamental component of blockchain systems, which settle the order of transactions. Many permissioned blockchain systems use leader-based Byzantine Fault Tolerant (BFT) consensus protocols. However, these leader-based BFT protocols are vulnerable to manipulation of transaction order by a malicious leader. Specifically, when transactions are visible to the leader, as in widely used systems such as Hyperledger Fabric, a malicious leader might exploit transaction dependencies and deliberately delay the execution of a transaction, making it invalid and effectively censoring it.To address this issue, we first analyze and refine the definition of these types of attacks, which we call "censorship by procrastination". We then propose a protocol designed to mitigate these attacks as well as other forms of transaction order manipulation. In addition, our protocol enables a wide range of legitimate transaction scheduling optimizations that can enhance performance, fairness, and other business objectives. Our protocol is straight-forward to implement and demonstrates optimal resilience. We present how this protocol can be integrated with SmartBFT, the consensus library run by Hyperledger Fabric. Finally, we evaluate the relative performance of SmartBFT with and without our censorship resistant protocol. Our results confirm that the overhead introduced by our protocol is reasonable and practical for real-world applications.