## 2 Background
### Blockchain  
Blockchain is a decentralized digital database carrying a chain of cryptographically signed blocks over a peer-to-peer network [28; 29]. The advancement of blockchain [30] surpassed the evolution of cryptocurrency and bitcoin with its unique significant features of maintaining transparency and preserving privacy in the distributed network [31; 32]. This leverages the power of utilizing blockchain technology in various sectors [33] including finance and trading [34; 35], supply chain management [36; 37], medicine and healthcare [38; 39], the IoT [40; 41; 42], and many other industrial services [43; 44; 45]. The blocks in the blockchain are generally arranged in a particular order, which contains the original data. Each block points to the preceding block using a hash value. The first block, called the genesis block, does not point to any block and, thus, is the parent/root block to the other subsequent child blocks. Each child holds a unique hash key, which is linked to the preceding block, also known as the parent block [46; 47]. The simplified architecture of blockchain and its components is illustrated in Figure 2.  
Figure 3 illustrates the connection of blocks and the Merkle tree connecting transactions to the block header. The digital signature is another significant component in the blockchain architecture. The user possesses two keys: public and private key. The transactions are secured by signing using the private key, and this digitally signed transaction is shared across the whole network. The public key is used to access the data present in the transaction by the authorized user. Hence, the generation of the digital signature is a two-step process, which includes the signing and verification process [48]. Figure 1: Schematic outline of the work. Following are the salient features of blockchain technology [49] that contribute to the powerful applications [50; 51] in this digital era:  
1. _Immuntality_--Blockchain is a permanent and unalterable network that promises no chance of changing the nodes in the network. The blockchain network has several nodes chained to each other, and every node maintains a copy of the digital ledger in the network. Hence, any transaction initiated in the network is authenticated and  
Figure 3: Connection of blocks and Merkle tree connecting transactions to a block header. Figure 2: Data communication using blockchain technology. verified to be included in the ledger. As a result, the data stored in any transaction are highly impossible to tamper with, as they are strictly protected by the nodes of the given network. This promotes the network to be highly transparent and secure, creating successful transactions with the consensus of all nodes in the network. Thus, it allows anyone to view the transactions, but does not give access to edit or modify the data in the transactions. 2. _Decentralization_--The framework is not operated under a single authority; rather, a collection of nodes is involved in managing and maintaining the blockchain network. The network is deployed on a peer-to-peer network, enabling each node to have a copy of the digital ledger, unlike the conventional banking system. Hence, the cost to hack such a decentralized network is more expensive, making it one of the most important features of blockchain technology. 3. _Smart contracts_--Blockchain executes the transactions in a faster way by applying the smart contract principle. Smart contracts are self-executing digital contracts that automatically execute the transactions when certain conditions and agreements are satisfied for the current transaction. 4. _Consensus protocol_--The consensus mechanism is the fault-tolerance mechanism in which nodes in the peer-to-peer network accede a common agreement about the current state of transactions in the network. This ensures the data reliability and trustworthiness of transactions among the nodes of blockchain. The Proof-of-Work (PoW) is the robust consensus protocol used widely in banking services and other applications.