## 1 Introduction  
Blockchain technology has recently acquired significant attention from academicians and the industry as evidenced by multiple applications, including copyright dispute resolution [1], product traceability [2], electronic voting [3], storage services [4], healthcare services [5], product tracking throughout the entire supply chain [6], and data management [7] in the internet-of-things (IoT) [8]. Prior researchers [9] predicted global expenditures on blockchain solutions to expand from 1.5 billion dollars in 2018 to 15.9 billion dollars annually by 2023. Blockchain technology provides pertinent data solutions through decentralization, collection, storage, and processing by recording transactions into respective blocks, which are appended to one another through cryptology to provide high security and validity levels. Specifically, each block is equipped with a reference number, known as a hash, to be attached to the subsequent block [10, 11] to maintain technical immutability, which requires all attached blocks to be edited if a specific block is to be amended [12]. Blockchain functions as a distributed database, which is regulated by a peer-to-peer network wherein all stakeholders (nodes) comply with a stipulated software protocol (consensus) to communicate and validate existing records or blocks [13, 14]. With every node possessing all copies of other blocks without centralized and official replicas, blockchain serves as a 'trustless' system without a third party to validate every performed transaction and facilitate interactions between corporations and clients [15; 16]. Immutability is a fundamental attribute of conventional blockchain technology to maintain high data integrity, which prevents transaction data modification while expediting the data auditing process [17; 18]. Notwithstanding, establishing procedures to authorize blockchain data redaction is also essential owing to three factors. Correspondingly, immutability might be abused for malevolent motives, such as illegal information storage and dissemination [19]. In [20], eight records related to sexual content were discovered in the Bitcoin blockchain, of which two comprised 274 child pornography website links, with 142 associated with darknet services. Therefore, the available arbitrary data storage is exploited due to child pornography, improper content, and illicit material violating intellectual rights [21]. With the misusage, users might be reluctant to adopt blockchain technology to avoid unintentional possession of illicit or inappropriate digital content and law violation. If the negative situation persists, thousands of blockchain nodes might be affected, consequently diminishing the Bitcoin ecosystem and functionality. Hence, continuously appending the latest digital information would not be feasible without illegal content being removed from the blockchain, which poses an essential prerequisite to further adoption by being obligatory for law enforcement agencies, including Interpol [22]. Another rationale for requiring the implementation of a redactable blockchain is triggered by imposed laws and regulations. According to the European General Data Protection Regulation (GDPR) [23; 24], users' data contain the \"right to be forgotten\", which allows all users to remove personal details and relevant duplicates while simultaneously amending personal data anonymously attached to former blocks [25]. Furthermore, blockchain immutability might not be pertinent to emerging blockchain-based applications, which adamantly request a certain flexibility level for data redaction. Examples include, but are not restricted to, the data stored on the blockchain, that may concern users' confidential and sensitive information, such as healthcare and insurance records [26; 27].