## 7 Redaction Mechanisms in Blockchain
### Redactable Blockchain-Based Chameleon
#### 7.1.1. Redactable Permissioned Blockchain
The modifier sends a key creation request, which includes the transaction to be modified, the required modification, and the key expiry time. The CA approves both the modifier's identity and request by signing the request with a conditional deposit to produce a token key before being sent to the modifier. A violated modifier triggers punishment by withdrawing agreement on the deposit. Meanwhile, it is committed to any conducted modification approvals wherein modifications are not stored immediately to facilitate checking by the miner. Only approved transactions will be stored in the blockchain public ledger. Nevertheless, security weakening, scalability and flexibility are less common due to CA centralization. Concurrently, computation overhead and malicious modifiers' privileges are not minimized as the deposit amount might be smaller than the damage cost. The authors in [67] have followed the previous suggestion by proposing a blockchain rewriting approach (CDEdit) that allows modifiers to acquire controllable redaction privileges and diversified editing types based on [66] frameworks. The CDEdit model, which is identified as a privilege token service (PTS), authorizes multilevel editing in on-chain permissioned blockchains, such as Hyperledger, to manage and disseminate redaction prerogatives. The entire process is triggered by modifiers upon sending a rewriting request. The modifiers are divided into four main categories, including one-time modifiers on the transaction and block level, and multi-time modifiers on the transaction and block level, without producing conflicts. Specifically, the process commences with requesting a token before receiving a token key (TK) which comprises the request of required rewriting times, expiry times, object indexes, and modifier public key (MPK) issued by the PTS. Before sending the TK to the modifier, a deposit is necessary to account for potential malicious actions after the PTS validates and signs the modifier's request. The PTS remains vulnerable due to centralization, which would negatively affect scalability with the presence of latency and computation overhead, and the absence of an effective punishment mechanism. [PERSON] et al. [68] adapted the [64]model by developing a revocable and traceable blockchain rewriting framework (ReTRACE) from the revocable CHET (RCHET) and revocable attribute-based encryption (RABE). Two primary dissimilarities exist wherein the RCHET hashing algorithm is a private procedure permitting only trapdoor holders to generate the CHs and subsequently conduct relevant adaptation, hence being inconsistent with the CH definition. Meanwhile, the hashing algorithm is intended to be a public procedure in the RABE to allow attribute revocation. Decentralization is attained via replacing ABE with MA-ABE; nonetheless, a public RABE is unrealistic as a user is required to acquire another decryption key through a secure medium when the user's attribute is repealed by the attribute authority (AA), although the group manager's (GM) attributes could not be revoked. Subsequently, [PERSON] et al. [69] developed a fine-grained blockchain rewriting mechanism to support hierarchical revocation via two stages. The first two data types in a mutable transaction, which were personal information (required to be recorded in a chain) and security-related details (senders, receivers, and transaction amounts), were considered. Users would be authorized by a semi-trusted regulator to amend existing personal records and modify security-related data. As the entire process concentrated on certain mutable transaction fields, the procedures would be more precise compared to the block-level and transaction-level blockchain redaction. Simultaneously, the regulator possessing a master key could repeal a user's rewriting privilege by generating an alternative subkey to disable the original key. Notwithstanding, the model did not establish a decentralization concept or indistinguishability. [PERSON] et al.