## 7 Redaction Mechanisms in Blockchain
### Redactable Blockchain-Based Chameleon
#### 7.1.1. Redactable Permissioned Blockchain
Meanwhile, block traceability is not supported, and accountability is excluded from the chameleon hash function, thus posing the possibility of malicious modifications. With the presence of the trapdoor key and centralized storage, distributed management is not feasible due to the missing decentralization characteristic. Subsequently, [PERSON] et al. [62] and [PERSON] et al. [63] enhanced the collision resistance feature of the CH corresponding to the Ateniese proposal [61]. However, better efficiency is achieved while similar construction is utilized. Both proposals sought trapdoor exposure prevention; meanwhile, they still suffer from similar [PERSON]'s proposal drawbacks. * Policy-based Chameleon Hash Function (PCH)  
Authority regulation to redact data in a fine-grained model, [PERSON] et al. [64] proposed the policy-based CH function (PCH) as shown in Figure 6.  
It is inspired by the CH with an ephemeral trapdoors (CHET) function [87], and cyphertexts policy attribute-based encryption (CP-ABE) [88] to serve as a hybridization function. The PCH function is associated with the user's attributes to control redaction privileges. The present study fulfilled two main questions by determining the individual authorized to perform a redaction and the specific information that could be redacted. Figure 5: [PERSON]’s proposal: randomness update in redacted block (edit). Figure 6: [PERSON] proposal. Specifically, users of private blockchains are permitted to perform redaction activities as their participation is authorized before becoming involved in the network. The CP-ABE provides the PCH function with high indistinguishability, collision, and resistance levels due to the security measures from indistinguishability under the non-adaptive and adaptive chosen cyphertexts attack (IND-CCA2), which prevents attackers from recovering trapdoor keys in conducting double-spending and tampering attacks. Meanwhile, the PCH chain allows transaction-level (TX-level) redaction without negatively impacting the system security, although collusion attacks are highly possible between participants by unifying personal attributes and updating or sharing access to an MT to be consistent with a specific strategy. Nonetheless, non-traceable evidence modifications are a security issue as the changes allowed by the PCH function would not be visible to other users. ABE is not guaranteed to encrypt valid one-time trapdoor keys without NIZK inclusion. The comprises of CHET base RSA in PCH is ineffective in terms of cost and time overhead. Other limitations of [PERSON]'s proposal [61], including non-accountability, distributed key management, trapdoor centralized storage, and central control of secret keys wherein decentralization remain unresolved, as in Figure 6 showing a delineated [PERSON]'s proposal [64]. [PERSON] et al. [65] propounded accountability to be the main contributing solution by proposing the policy-based CH function with black-box accountability (PCHBA) by combining the CHET, the CP-ABE, and the digital signature. The PCHBA is advantageous in prohibiting a transaction modifier to perform malicious rewriting with the accountability property to halt any exploitation form of rewriting privilege. Nonetheless, the PCHBA violates user anonymity as the function supports key generation by central authority, while privilege revocation and decentralization are not considered. PCHBA deprives an effective penalty mechanism to stop malicious redaction activities. As such, [PERSON] et al. [66] proposed the k-time modifiable and epoch-based redactable blockchain (KERB), which consists of a monetary penalty to regulate rewriting privileges and punish malevolent demeanors. The proposal improved the [64] algorithm by ensuring sufficient accountability and traceability to account for the lack of punishment mechanisms and negligence of collusion attack possibility. The KERB comprises the central authority (CA), the miner, and the modifier.