The immutability of blockchain, while essential for ensuring data integrity and traceability, presents challenges in scenarios requiring data modification, such as regulatory compliance with the General Data Protection Regulation (GDPR) and correction of erroneous transactions. To address this, we propose TAR−PCH, a novel chameleon hash scheme that enables fine-grained, policy-controlled rewriting of blockchain data under cloud-assisted environments. Our construction extends the Policy-Based Chameleon Hash (PCH) framework by incorporating partial policy hiding, user-level and attribute-level revocation, and traceability of malicious users. Leveraging efficient attribute-based encryption and white-box tracing, TAR−PCH enforces strong access control and accountability while outsourcing costly operations to the cloud. We formalize its security via indistinguishability and collision-resistance models, and our implementation demonstrates practical performance and scalability compared to existing schemes.

Policy-based chameleon hash (PCH) is one of the techniques used for rewriting transaction-level data stored in blockchains. This technique integrates the access policy of the attribute-based encryption (ABE) in the transactions and only allows users with attributes set satisfying the access policy to modify the transactions. However, some operations in the PCH-based rewritable blockchain solution require high computational cost which may impact the performance of user systems, especially on resource-constrained devices. To solve this problem, we propose an online/offline rewritable blockchain with auditable outsourced computation (OO-RB-AOC) scheme. We utilize the ring signature to ensure the credibility of multiple attribute authorities, and adopt the online/offline technique for generating the hash of the rewritable transaction. In addition, expensive computations (e.g., pairings) required for rewriting the transactions can be outsourced to the clouds. The users can rest assured that the computations from the clouds are correct with the audit mechanism of the proposed scheme. On the other hand, the proposed scheme offers a desirable feature for commercial application where the clouds can limit the number of outsourced requests according to the subscription of the users. We also prove the security of the proposed OO-RB-AOC scheme. Finally, we present a theoretical comparison and experimental analysis of the proposed scheme.

One of the solutions for rewriting blockchain data is to replace the standard hash function with a chameleon hash function. This enables users with the trapdoor to generate hash collision for rewriting operation. However, such rewritable blockchain solution may allow users to have unlimited rewriting permissions. This is undesirable as rewritable blockchains should still maintain a certain degree of immutability. In this paper, we propose a fine-grained one-time rewritable blockchain scheme that supports traitor tracing and bilateral access control. The proposed solution is the first that combines one-time chameleon hash function and traceable attribute-based encryption to realize one-time rewriting blockchain at the same time tracking for malicious users who leak the private key. Furthermore, our solution offers bilateral access control by integrating with fog computing. We also verify the performance of the proposed scheme with experimental analysis.