  Modern Byzantine Fault-Tolerant State Machine Replication (BFT-SMR) solutions
focus on reducing communication complexity, improving throughput, or lowering
latency. This work explores the energy efficiency of BFT-SMR protocols. First,
we propose a novel SMR protocol that optimizes for the steady state, i.e., when
the leader is correct. This is done by reducing the number of required
signatures per consensus unit and the communication complexity by order of the
number of nodes n compared to the state-of-the-art BFT-SMR solutions.
Concretely, we employ the idea that a quorum (collection) of signatures on a
proposed value is avoidable during the failure-free runs. Second, we model and
analyze the energy efficiency of protocols and argue why the steady-state needs
to be optimized. Third, we present an application in the cyber-physical system
(CPS) setting, where we consider a partially connected system by optionally
leveraging wireless multicasts among neighbors. We analytically determine the
parameter ranges for when our proposed protocol offers better energy efficiency
than communicating with a baseline protocol utilizing an external trusted node.
We present a hypergraph-based network model and generalize previous fault
tolerance results to the model. Finally, we demonstrate our approach's
practicality by analyzing our protocol's energy efficiency through experiments
on a CPS test bed. In particular, we observe as high as 64% energy savings when
compared to the state-of-the-art SMR solution for n=10 settings using BLE.
