  Randomized smoothing has recently attracted attentions in the field of
adversarial robustness to provide provable robustness guarantees on smoothed
neural network classifiers. However, existing works show that vanilla
randomized smoothing usually does not provide good robustness performance and
often requires (re)training techniques on the base classifier in order to boost
the robustness of the resulting smoothed classifier. In this work, we propose
two cost-effective approaches to boost the robustness of randomized smoothing
while preserving its clean performance. The first approach introduces a new
robust training method AdvMacerwhich combines adversarial training and
robustness certification maximization for randomized smoothing. We show that
AdvMacer can improve the robustness performance of randomized smoothing
classifiers compared to SOTA baselines, while being 3x faster to train than
MACER baseline. The second approach introduces a post-processing method EsbRS
which greatly improves the robustness certificate based on building model
ensembles. We explore different aspects of model ensembles that has not been
studied by prior works and propose a novel design methodology to further
improve robustness of the ensemble based on our theoretical analysis.
