## 9 Open Challenges and Future Research Direction  
Recent advancements in designing mutable blockchains are still immature, and consequent modeling redaction techniques require further investigations. This section is dedicated to enriching interested researchers' knowledge and shaping future research direction through open issues listed below:  
* **Permissionless settings. ** Permissioned and consortium blockchain concepts have adopted most of the proposed solutions where permissionless settings remain a challenge due to their openness and unrestricted characteristics and, hence, it is still unclear how to solve chameleon-based redaction in public blockchain. However, there are several suggestions for a non-chameleon method which proved to be inefficient due to time overhead. Non-chameleon is out of the scope of this research. * **Redaction exceptional circumstances. ** Redaction in total is a delicate case due to violating major blockchain immutability features. The balance requires special supplements, and careful procedures in order to succeed in redaction without any contradiction. * **Revocation scalability consequences. ** Centralization revoking mechanisms lack efficiency and effectiveness. Consequently, it overwhelmingly poses cost, time, communication overhead and even other equipment that might be required. * **Trapdoor key exposure. ** The trapdoor key is the main weakness of the chameleon hashing function; meanwhile, the entire collision being indistinguishable relies on its secrecy. * **Revocation centralization authorities. ** Studies in the current domain have employed central authorities to perform redaction; however, centralization is considered a drawback and decentralization is highly recommended. * **Punishment methodologies. ** Punishment has been replaced by accountability where violators remain safe without any further actions taken against them. * **IOT-based redaction in blockchain performance. ** Scalability is an ineffective factor due to a lack of blockchain redaction in the IoT domain wherein performance criteria are still low due to edge device limitations not being considered. According to authentic requirements, blockchain-based IOT/IIOT designs cannot provide networking resources. Current designs mainly suffer from a dispute among network resources, security, and redaction. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Security Properties**} & \multicolumn{6}{c}{**Challenges**} \\ \hline  & & & & & & & & & & & & \\ \hline
[95], **(2019)** & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & & & \(\surd\) & & P \\ \hline
[96], **(2020)** & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & & \(\surd\) & & P \\ \hline
[97], **(2017)** & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & & & \(\surd\) & & & \(\surd\) & M \\ \hline
[98], **(2020)** & & & & \(\surd\) & \(\surd\) & \(\surd\) & & & & & M \\ \hline
[99], **(2019)** & & & & & & & & \(\surd\) & & & P \\ \hline
[100], **(2020)** & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & & \(\surd\) & & & P \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Non-chameleon redaction approaches challenges comparisons. **The balance between accountability and anonymity. ** Privacy is a legal right to preserve identity concealment. Proposals mainly prioritize accountability over personal privacy. However, GDPR legislation strictly states that hidden identities must be maintained which is acutely confronted by the recent redaction mechanisms concepts. * **Rewriting flexibility limiting. ** Absolute power offered to the rewriting modifiers sabotaged data integrity and confined rewriting abilities to themselves. * **Redactor granting privileges agreement. ** The cooperation between owners and modifiers demands prearranged agreements.