Blockchain Scalability and Security: Communications Among Fast-Changing Committees Made Simple

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.