Fraud and Data Availability Proofs: Maximising Light Client Security and Scaling Blockchains with Dishonest Majorities
This work addresses the challenge of scaling blockchains securely for users and developers by allowing light clients to operate without trusting a majority of block producers, which is crucial for applications like sharding or larger blocks.
The paper tackles the problem of light client security in blockchains by introducing a fraud and data availability proof system that eliminates the need for an honest-majority assumption, instead relying on weaker assumptions about honest nodes rebroadcasting data, enabling on-chain scaling while ensuring data validity and availability.
Light clients, also known as Simple Payment Verification (SPV) clients, are nodes which only download a small portion of the data in a blockchain, and use indirect means to verify that a given chain is valid. Typically, instead of validating block data, they assume that the chain favoured by the blockchain's consensus algorithm only contains valid blocks, and that the majority of block producers are honest. By allowing such clients to receive fraud proofs generated by fully validating nodes that show that a block violates the protocol rules, and combining this with probabilistic sampling techniques to verify that all of the data in a block actually is available to be downloaded, we can eliminate the honest-majority assumption, and instead make much weaker assumptions about a minimum number of honest nodes that rebroadcast data. Fraud and data availability proofs are key to enabling on-chain scaling of blockchains (e.g. via sharding or bigger blocks) while maintaining a strong assurance that on-chain data is available and valid. We present, implement, and evaluate a novel fraud and data availability proof system.