GearBox: An Efficient UC Sharded Ledger Leveraging the Safety-Liveness Dichotomy

Published in

Concordium

2 min readMar 19, 2021

Bernardo David¹, Bernardo Magri² , Christian Matt³ , Jesper Buus Nielsen² , and Daniel Tschudi³

Sharding is an emerging technique to overcome scalability issues on blockchain based public ledgers. Without sharding, every node in the network has to listen to and process all ledger protocol messages. The basic idea of sharding is to parallelize the ledger protocol: the nodes are divided into smaller subsets that each take care of a fraction of the original load by executing lighter instances of the ledger protocol, also called shards. The smaller the shards, the higher the efficiency, as by increasing parallelism there is less overhead in the shard consensus.

In this vein, the authors propose a novel approach that leverages the sharding safety-liveness dichotomy. They separate the liveness and safety in shard consensus, allowing us to dynamically tune shard parameters to achieve essentially optimal efficiency for the current corruption ratio of the system. They start by sampling a relatively small shard (possibly with a small honesty ratio), and carefully trade-off safety for liveness in the consensus mechanism to tolerate small honesty without losing safety. However, for a shard to be live, a higher honesty ratio is required in the worst case. To detect liveness failures, the researchers use a so-called control chain that is always live and safe. Shards that are detected to be not live are resampled with increased shard size and liveness tolerance until they are live, ensuring that all shards are always safe and run with optimal efficiency. As a concrete example, considering a population of 10K parties, 30% corruption and 60-bit security, our design permits shards of size 200 parties in contrast to 6K parties in previous designs.

Moreover, in this highly concurrent execution setting, it is paramount to guarantee that both the sharded ledger protocol and its sub protocols (e.g., the shards) are secure under composition. To prove the security of our approach, the authors present ideal functionalities capturing a sharded ledger as well as ideal functionalities capturing the control chain and individual shard consensus, which needs adjustable liveness. They further formalize our protocols and prove that they securely realize the sharded ledger functionality in the UC framework.