“Reparo: Publicly Verifiable Layer to Repair Blockchains” Accepted at Financial Cryptography 2021

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The paper „Reparo: Publicly Verifiable Layer to Repair Blockchains“ by Sri Aravinda Krishnan ThyagarajanAdithya Bhat, Bernardo MagriDaniel Tschudi and Aniket Kate has been accepted at Financial Cryptography and Data Security 2021.

Below you will find the abstract of the paper.

Although blockchains aim for immutability as their core feature, several instances have exposed the harms with perfect immutability. The permanence of illicit content inserted in Bitcoin poses a challenge to law enforcement agencies like Interpol, and millions of dollars are still being lost in buggy smart contracts in Ethereum. All the existing proposals follow a build-new-chain approach for redactions, and cannot be integrated into existing running blockchains.

This work demonstrates that the build-new-chain approach for blockchain redactions is not necessary. We present Reparo, a publicly verifiable layer on top of any blockchain to perform repairs, ranging from fixing buggy contracts to removing illicit contents from the chain. We present an efficient instantiation of Reparo over Ethereum (with proof of work based consensus) for repairing smart contract bugs. In this protocol, any Ethereum user may propose a repair and a deliberation process ensues resulting in a decision that complies with the repair policy of the chain and is publicly verifiable. A repair operation (for instance, fixing a bug in a contract) is then performed according to the repair proposal and the state of Ethereum is updated accordingly. Reparo’s advantages are multi-fold:

(i) Since Reparo follows a layer design, it helps facilitate additional functionalities for Ethereum while maintaining the same provable security guarantees;
(ii) Reparo can be easily tailored to different consensus requirements (like proof of stake), does not require heavy cryptographic machinery, and thus, can be integrated with other existing blockchains (such as Bitcoin, Cardano) as well.

We evaluate Reparo with Ethereum mainnet and show that the cost of fixing several prominent smart contract bugs is almost negligible. For instance, the cost of repairing the prominent Parity Multisig wallet bug with Reparo is as low as 0.000000018% of the Ethers that can be retrieved after the fix.