Abstract
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 GDPR in the EU), 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. In their seminal work, Ateniese et al. (IEEE EuroS&P 17) 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 functions (i.e., chameleon hash functions) and secure multi-party computation. We observe that the redaction technique of Ateniese et al. does not take into account the possibility that the blockchain supports 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, and, more in general, the state of a smart contract inconsistent with the content of transactions. We find this choice rather limiting since decentralized and publicly verifiable computation guaranteed by smart-contract-enabled blockchains is necessary for modern (i.e., Web3) applications. 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 to realize what we call a proof-of-consistency.