  For permissionless blockchains, scalability is paramount. While current
technologies still fail to address this problem fully, many research works
propose sharding or other techniques that extensively adopt parallel processing
of transactions. In these approaches, a potentially large number of committees
of nodes independently perform consensus and process new transactions. Hence,
in addition to regular intra-committee communication, (1) new transactions have
to be delivered to the right committee, (2) committees need to communicate to
process inter-shard transactions or (3) to exchange intermediate results. To
contrast slowly adaptive adversaries, committees should be frequently changed.
However, efficient communication to frequently-changing committees is hard.
  We propose a simple approach that allows us to implicitly select committee
members and effectively deliver messages to all members of a specific
committee, even when committees are changed frequently. The aim of our design
is to provide a committee selection procedure and a committee-targeted
communication primitive to be applied in most of the scalable blockchain
architectures that are currently proposed in literature. We provide a
theoretical proof of the security of our approach and first experimental
results that shows that our approach might be feasible in practice.
