  Consensus is one of the most fundamental distributed computing problems. In
particular, it serves as a building block in many replication based
fault-tolerant systems and in particular in multiple recent blockchain
solutions. Depending on its exact variant and other environmental assumptions,
solving consensus requires multiple communication rounds. Yet, there are known
optimistic protocols that guarantee termination in a single communication round
under favorable conditions.
  In this paper we present a generic optimizer than can turn any consensus
protocol into an optimized protocol that terminates in a single communication
round whenever all nodes start with the same predetermined value and no
Byzantine failures occur (although node crashes are allowed). This is
regardless of the network timing assumptions and additional oracle capabilities
assumed by the base consensus protocol being optimized.
  In the case of benign failures, our optimizer works whenever the number of
faulty nodes $f<n/2$. For Byzantine behavior, our optimizer's resiliency
depends on the validity variant sought. In the case of classical validity, it
can accommodate $f<n/4$ Byzantine failures. With the more recent external
validity function assumption, it works whenever $f<n/3$. Either way, our
optimizer only relies on oral messages, thereby imposing very light-weight
crypto requirements.
