SEREDACT: Secure and Efficient Redactable Blockchain with Verifiable Modification

The immutability of blockchains is an important security feature, but applications and studies have shown that it poses some problems. For instance, harmful information and vulnerable programs can be permanently stored on public blockchains such as Bitcoin and Ethereum, causing continuous damage. Therefore, researchers proposed the redactable blockchain to delete or modify those harmful data. Existing schemes usually adopt the Chameleon hash function (CHF) to keep the block hash unchanged so that other blocks remain unaffected. However, these schemes suffer from two security problems: (i) (unknown-version) users cannot determine whether a received block is the up-to-date version because different versions have the same hash; and (ii) (lazy-redaction) miners have no motivations to update historical blocks, causing continuous spreading of data which should have been discarded. To solve the problems, we propose SEREDACT, a secure and efficient redactable blockchain protocol with verifiable modification. Specifically, we design a Merkle tree-based verification mechanism with efficient dynamic updating that supports quick version checks and forcible modification updates, and further integrate it with restricted redaction policies to guarantee security. Our security and performance analyses show that SEREDACT has adequate security as a redactable blockchain protocol and retains close efficiency compared with the immutable blockchain.