Paper 2024/1299

Permissionless Verifiable Information Dispersal (Data Availability for Bitcoin Rollups)

Abstract

Rollups are special applications on distributed state machines (aka blockchains) for which the underlying state machine only logs, but does not execute transactions. Rollups have become a popular way to scale applications on Ethereum and there is now growing interest in running rollups on Bitcoin. Rollups scale throughput and reduce transaction costs by using auxiliary machines that have higher throughput and lower cost of executing transactions than the underlying blockchain. State updates are periodically posted to the underlying blockchain and either verified directly through succinct cryptographic proofs (zk rollups) or can be challenged for a defined period of time in a verifiable way by third parties (optimistic rollups). However, once computation is removed as a bottleneck, communication quickly becomes the new bottleneck. The critical service the underlying blockchain provides in addition to verification is data availability: that necessary data can always be recovered upon request. While broadcasting transaction data is one way to ensure this, it requires communication blowup linear in the number of participating nodes. Verifiable information dispersal (VID) systems achieve sublinear blowup in the same participation model and the same security assumptions as Ethereum, where all nodes have a strong public-key identity. It was not known how to do so in the same permissionless model as Bitcoin, where participants are unauthenticated and participation is dynamic. We construct a VID system that is secure under the same model as Bitcoin, with one minimal additional requirement on the existence of reliable participants. Our system uses a state machine replication (SMR) protocol (e.g., Bitcoin) as a black box, and is therefore backward compatible. We implemented the system on top of Bitcoin core with the Regression Test Network (regtest), and our analysis shows that it reduces communication costs by more than 1,000x and latency by more than 10x.