  Byzantine fault tolerant (BFT) state machine replication (SMR) is an
important building block for constructing permissioned blockchain systems. In
contrast to Nakamoto Consensus where any block obtains higher assurance as
buried deeper in the blockchain, in BFT SMR, any committed block is secure has
a fixed resilience threshold. In this paper, we investigate strengthened fault
tolerance (SFT) in BFT SMR under partial synchrony, which provides gradually
increased resilience guarantees (like Nakamoto Consensus) during an optimistic
period when the network is synchronous and the number of Byzantine faults is
small. Moreover, the committed blocks can tolerate more than one-third (up to
two-thirds) corruptions even after the optimistic period. Compared to the prior
best solution Flexible BFT which requires quadratic message complexity, our
solution maintains the linear message complexity of state-of-the-art BFT SMR
protocols and requires only marginal bookkeeping overhead. We implement our
solution over the open-source Diem project, and give experimental results that
demonstrate its efficiency under real-world scenarios.
