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Fraud Proofs

Overview

Following is an excerpt from Fraud and Data Availability Proofs:Detecting Invalid Blocks in Light Clients 1.

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. Instead of validating blocks, 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 so that fraud can be detected, we can eliminate the honest-majority assumption for block validity, 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 while maintaining a strong assurance that on-chain data is available and valid. We present, implement, and evaluate a fraud and data availability proof system.

Here is an overview of how NEAR bridge uses this in an optimistic approach

The leading NEAR Ethereum Bridge today Near Rainbow Bridge uses an optimistic approach. Following is an excerpt from NearOnEthClient 2.

we adopt the optimistic 3 approach where NearOnEthClient verifies everything in the NEAR header except the signatures. Then anyone can challenge a signature in a submitted header within a 4-hour challenge window. The challenge requires verification of a single Ed25519 signature which would cost about 500k Ethereum gas (expensive, but possible).

Footnotes

  1. Fraud and Data Availability Proofs:Detecting Invalid Blocks in Light Clients: Fraud and data availability proofs are key to enabling on-chain scaling of blockchains while maintaining a strong assurance that on-chain data is available and valid. ↩

  2. NEAR: ETH-NEAR Rainbow Bridge: a bridge, called Rainbow Bridge, to connect the Ethereum and NEAR blockchains. ↩

  3. Optimistic Contracts: contracts that accept all information as fact until proven to be non-factual. This allows for a reduction in the cost of verifying data, as on-chain verification would only be necessary when one is sure that the data is false. ↩