Brief Note: Fast Authenticated Byzantine Consensus. (arXiv:2102.07932v2 [cs.DC] UPDATED)

Byzantine fault-tolerant (BFT) state machine replication (SMR) has been
studied for over 30 years. Recently it has received more attention due to its
application in permissioned blockchain systems. A sequence of research efforts
focuses on improving the commit latency of the SMR protocol in the common good
case, including PBFT with $3$-round latency and $ngeq 3f+1$ and FaB with
$2$-round latency and $ngeq 5f+1$. In this paper, we propose an authenticated
protocol that solves $2$-round BFT SMR with only $ngeq 5f-1$ replicas, which
refutes the optimal resiliency claim made in FaB for needing $n geq 5f+1$ for
$2$-round PBFT-style BFT protocols. For the special case when $f=1$, our
protocol needs only $4$ replicas, and strictly improves PBFT by reducing the
latency by one round (even when one backup is faulty).