## 7 Redaction Mechanisms in Blockchain
### Non-Chameleon Hash-Based Redactable Blockchain
#### 7.2.1 Consensus-Based Voting Redaction Approach (CVRA)  
The CVRA leverages standard regulations to concur on the eventual condition of redacted transactions or blocks via on-chain voting. A redaction would only be approved when all honest nodes consent to the altered state. Past studies [101] employed the hard fork to amend the information history, resolve crucial security risks in codes, include additional functionality, restore previous transactions, and overcome negative hacking consequences. For instance, Ethereum applied the hard fork to restore past transactions in recovering substantial funds lost to \"The DAO\" attack. When a fork simultaneously appears in at least two block versions of the same height, an alternative chain will be produced. Although accidental forks could be resolved via identical consensus principles, attackers' chains are more difficult to be removed due to the longer series than the honest version (51% attack). Consequently, the hard fork adhering to the latest consensus principles would generate an irreversible sequence independent of the original chain, which is typically employed to divide blockchain cryptocurrency. For example, Bitcoin Gold and Bitcoin Cash were separated from Bitcoin [102]. Nonetheless, several limitations exist as the hard fork would not erase exact data history as both original and alternative chains would be maintained by respective nodes. Moreover, a high cost is incurred as the procedure consumes significant computation bandwidth. [PERSON] et al. [95] suggested authorizing data redaction in the public blockchain with the CV (consensus voting) sequence expanding the block header structure to provide additional fields encompassing previous states or hash values, which enabled connectivity between redacted and subsequent blocks. All blocks are connected by two hash chains to represent the latest prevHash and prior hashes, with the following block constantly being attached to the previous block after referring to the prior condition. For multiple redactions, the old state consists of all states of all revisions. Moreover, the CV series adequately sustains consistency through honest nodes, holding a unified monitor of redaction activities by publicly validating before voting. The redaction request on a specific transaction would be announced in the peer-to-peer (P2P) network, where miners securing a subsequent block could vote for approving a valid revision while recording the redacted block hash in personal blocks. The revision would be approved when the editing request on a particular block received sufficient votes. Resultantly, honest nodes would update personal copies with the performed redaction. Scalability and efficiency degrees are low in the process owing to the significant amount of time required for approval of valid redactions. [PERSON] et al. [103] formulated an amendable blockchain protocol in two hash chains, wherein the standard chain stored the original data, whereas the correction chain comprised numerous blocks to store correction data. The CV approach was also applied to perform a decentralized decision on requested corrections, in which a correction block consisting of the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Schema**} & \multirow{2}{*}{**Methods**} & **Security** & **Setting** & **Redaction** \\  & & **Assumption** & **(Blockchain Type)** & **Type** \\ \hline
[95], **(2019)** & CVRA & N/A & Public & TL \\ \hline
[96], **(2020)** & CVRA & N/A & Private/Public & TL \\ \hline
[97], **(2017)** & MTRA & N/A & Private/Public & TL \\ \hline
[98], **(2020)** & MTRA & N/A & Private & TL \\ \hline
[99], **(2019)** & PRA & N/A & Public & BL \\ \hline
[100], **(2020)** & PRA & N/A & Public & BL \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Non-chameleon redaction approaches outlines. actual correction data replaced the redacted block with the same height in the standard chain after the voting session. Nevertheless, several challenges persist, including a long voting period and poor redaction efficiency.