## 7 Redaction Mechanisms in Blockchain
### Redactable Blockchain-Based Chameleon
#### 7.1.1. Redactable Permissioned Blockchain
[70] established a revocable policy CH function (RPCH) adapted from the PCH [64] to aid in fine-grained and revocable blockchain rewriting approaches. The RPCH development referred to CHET based on the Rivest, Shamir, and Adleman (RSA) [89] proposal with the RABE method [90], and the fast attribute-based message encryption (FAME) process [91]. The RPCH also did not include a blockchain decentralization feature in its design via permitting a trusted party to practically repeal a chameleon trapdoor possessor's redaction prerogative, outsource the decoding process, and transmit confidentiality without another semi-trusted entity however CHET performs based on RSA, which affects scalability issues. Meanwhile, [PERSON] et al. [71] and [PERSON] et al. [72] proposed a multi-authority policy-based CH (MAPCH) function, wherein the CA in the [64] model is replaced with multiple authorities to manage the attributes applied to a permissioned blockchain. The performance in both proposals has been improved due to the decentralized technique proposed, however the reliance on RSA remains a scalability drawback and the absence of revocation requires further investigation. [PERSON] et al. [73] suggested a hybrid blockchain rewriting process both online and offline via an auditable outsource computation (OO-RB-AOC) scheme, which was adapted from [64]. The hybrid model is an enhancement to produce two PCH types, namely online and offline. Particularly, high-value transaction operations could be performed offline to provide adequate credibility by incorporating multiple authorities to generate rewriting keys. Beneficiaries who acquired the keys would be verified on whether the keys were issued by legitimate authorities. The previous model has been built based on ring signature [92] and is employed to create a rewriting key in which external resources could be continuously applied due to the limited device capacity. Nevertheless, the limitations include the lack of a pertinent mechanism for revocation and misbehavior observation. Meanwhile, [PERSON] et al. [74] established a fine-grained and governable redactable blockchain model to allow destructive information forced erasure. Specifically, the transaction creator possesses a delicate regulation over selective individuals to conduct data redaction by specifying the components to be edited, after receiving sufficient miners' votes. All miners could also disclose the block index with detrimental data generated by malevolent users to be subsequently recompensed after authoritatively erasing the deleterious information based on the index. Malevolent users would be prohibited from performing further transactions if the penalty was not paid as miners' compensation within a stipulated period. Moreover, the fine-grained framework facilitates additional information and unexpended transaction output (UTXO) redaction concurrently. Nonetheless, revocation is unavailable for any misdemeanors and decentralization is not recognized with the presence of a trusted authority, significantly influencing the model's performance. * Distributed Chameleon Hash  
[PERSON] et al. [75] have proposed the inclusion of the distributed key management function by using distributed CH function and trusted execution environment (TEE) [93]. Distributed key management is employed to guarantee high trapdoor security levels, which could be strengthened by the TEE to elevate asset confidentiality and integrity degree in a secure TEE enclave. Nevertheless, as the rewriting procedure is performed throughout the blockchain level, weak security is contributed by the exclusion of collusion, accountability, and tractability. [PERSON] et al. [76] further improved the existing CH function. In the present study, the development was based on enhancing the collision resistance characteristic which was perceived to be highly vital to managing redactable blockchain.