## 3 Background
### Blockchain  
In a study by [PERSON] [6], BT was initially described in the form of Bitcoin. Blockchain is a distributed digital storage platform that captures transaction details in a ledger and stores them across several nodes in a transparent manner [7; 8]. Blockchain is also known as a public ledger of transactions because all participants in the transaction disseminate and disclose the data in a bundle rather than retaining the record of transactions only on the central server, as is the current technique in data transactions [9].  
A blockchain is a chronological series of blocks that includes a list of complete and valid transaction data. A reference (hash value) links each block to the one before it, producing a blockchain. The parent block is the block that comes before a specific block, and the genesis block is the first block [10]. A blockchain functions as an immutable timestamp ledger of blocks that can be shared between all participants in the network, so there is no need for a central authority [11]. Peer-to-peer networks employ this technology to distribute and store data in a distributed manner [12]. A blockchain is helpful for financial transactions and in various other situations. Decentralized IoT includes identity-based PKI, a decentralized supply chain, decentralized evidence of document existence, decentralized storage, and so on [13]. BT can be applied to various non-financial industries, including IoT, healthcare, education, and a variety of other non-financial start-ups [14].  
All data are stored in blocks in the blockchain [15]. However, when a large amount of data is saved in blocks, the entire network's pace is reduced, personal information cannot be recorded, entered data cannot be removed, and all data are visible to all participants--a privacy infringement that violates the Personal Information Protection Act [16].  
Using on-chain and off-chain concepts, blockchain overcomes this problem [17]. All network actions recorded on the blockchain are referred to as on-chain [18], and all actions that occur outside of the blockchain are referred to as off-chain [19]. This method can alleviate the decreased pace and the challenge of protecting sensitive information, such as personal data, because it does not entail writing directly to the blockchain [20]. Furthermore, data saved off-chain are validated on-chain to assure data forgery is prevented [21].  
Blockchain's unique design is based on three key components: cryptography, peer-to-peer networks, and consensus mechanisms. These three characteristics provide the blockchain a high level of power, allowing it to be used in various applications (not just for digital cash). The following is a list of blockchain technology's features [22]:  
* Decentralization: The blockchain's information is copied and dispersed by the network's nodes, which can independently validate the blockchain without needing a central authority.
* Immutability (tamper-proof): The neighbors keep a permanent record of transactions (ledger). As a block is cryptographically locked in the ledger once it is upended, it cannot be updated. This instills confidence in the transaction record.
* Transparency: The ledger contains a complete record of all transactions, and anyone can examine and audit transactions on the blockchain because it is an open file. This provides provenance, allowing asset lifetimes to be traced.
* Time-stamped: Blockchain's cryptographic technique connects blocks in a chronological chain, allowing for a trial of the underlying transactions. Cryptography in blockchain ensures the network's security [23]. All blocks of the blockchain are linked to each other so that each block is linked to the one before it and the next one, making it difficult for an attacker to edit any record. Doing so would also require changing the records or blocks related to manipulated records. This is nearly impossible in an extensive network with many blocks in a blockchain [24].