Brief Announcement: Byzantine Agreement, Broadcast and State Machine Replication with Optimal Good-Case Latency

This paper investigates the problem \textit{good-case latency} of Byzantine agreement, broadcast and state machine replication in the synchronous authenticated setting. The good-case latency measure captures the time it takes to reach agreement when all non-faulty parties have the same input (or in BB/SMR when the sender/leader is non-faulty) and all messages arrive instantaneously. Previous result implies a lower bound showing that any Byzantine agreement or broadcast protocol tolerating more than $n/3$ faults must have a good-case latency of at least $\Delta$ \cite{synchotstuff}, where $\Delta$ is the assumed maximum message delay. Our first result is a matching tight upper bound for a family of protocols we call $1\Delta$. We propose a protocol $1\Delta$-BA that solves Byzantine agreement in the synchronous and authenticated setting with optimal good-case latency of $\Delta$ and optimal resilience $f