Data Redaction in Smart-Contract-Enabled Permissioned Blockchains

Balancing immutability and compliance with regulations stands as a significant challenge in the realm of blockchain technology applications. Due to the increase of data-protection requirements (e.g., the General Data Protection Regulation (GDPR) in the European Union), it is essential to address the problem of deleting data from a blockchain without compromising the security and transparency of the blockchain itself. Several works proposed techniques to address the data redaction problem, and the most effective ones are mainly applicable to permissioned blockchains. In their seminal work, Ateniese et al. [EuroS&P 2017] were the first to propose a redactable blockchain. Their approach focuses on permissioned blockchains and they showed how to change the content of a transaction without breaking the chaining among blocks by using special cryptographic hash function (i.e., chameleon hash functions) and protocols for secure multi-party computation to make the required distributed computations. We observe that the redaction technique of Ateniese et al. does not take into account the possibility that the blockchain includes smart contracts and that a redaction of a transaction might leave inconsistencies in the logic of the contracts, making some remaining non-redacted transactions invalid. We find this decision rather limiting since decentralized and publicly verifiable computation guaranteed by smart-contract-enabled blockchains is indeed necessary for modern applications. In order to overcome the above limitations of the applicability of the redaction techniques of Ateniese et al., we propose a redaction technique with wider applicability that leverages succinct non-interactive arguments of knowledge (SNARKs) to realize what we call a proof-of-consistency. A SNARK will be computed to show that the current state of a smart contract is consistent with respect to a previous state since there existed transactions, now redacted, that produced the current state from a former publicly verifiable state. Therefore, a successful verification of such a SNARK allows to maintain the consistency of the non-redacted transactions with the state of the smart contract, and the resulting blockchain maintains public verifiability and consistency in the presence of redacted transactions.