Immutability has always been a core attribute of blockchain to ensure a trusted public ledger. However, in recent years, many malicious actions on blockchain have raised controversy over strict immutability. As a practical matter, it is necessary to design redactable blockchain protocols in some cases. Previous studies have proposed methods to implement redaction features in permissionless blockchains. However, in these protocols, parties can refuse to accept redacted blocks without affecting their mining. It makes these protocols lack feasibility in practice. In this paper, we present Chainknot, a redactable blockchain protocol with forced synchronization. We use the incentives of permissionless blockchain to effectively facilitate the review of a redaction proposal, and design a novel blockchain structure that ensures the acceptance of modified blocks by parties. In addition, we present instantiations of Chainknot on the PoW-based and PoS-based blockchain systems, with further improvements for efficiency and security, respectively. The experimental results show that the additional latency our protocol imposes on the original immutable blockchain system is negligible (less than 4%).
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