2.4.1. PoW

During mining, a new block is created by computing the block's cryptographic hash. To prove its validity on the blockchain, a block hash must meet certain conditions. The

Bitcoin blockchain, for example, starts each hash block with four trailing zeroes. Given that block data, which are transactional data, cannot be changed, the miner must modify the predefined hash pattern at every occurrence. The two network partners compete for the right nonce to create a valid block hash. Initially, the miner who seeks a solution attaches the block to the chain. As a reward for the miner's efforts, the system produces a certain number of coins and provides the newly produced coins to the miner.

The PoW mechanism entirely relies on the computer power of the miner. The more computing power a miner has, the greater the chance of finding blocks and earning rewards [46]. In PoW consensus, half of the network's nodes are assumed to remain trustworthy. As a result, this consensus is vulnerable because more than half the hashing power is owned by a single party. The cost of resources and hardware is one of the significant disadvantages of PoW. Several studies have reported that the energy consumption of Bitcoin mining is considerably higher than the energy consumption of 159 countries [47]. By contrast, the mining requirements and the mining time can differ depending on the algorithm used by each cryptocurrency. PoW mining is relatively slower than other consensus protocols. Given that a small number of mining pools dominate the Bitcoin network, attacks on these pools may result in severe disruptions. Recent attacks have demonstrated that PoW is vulnerable to 51% attacks. Low-hacking crypto coins based on PoW consensus are susceptible to 51% attacks because the requisite hash is easy to obtain. With the appropriate budget, the P+ epsilon attack can be conducted at no cost [48]. In addition, the researchers in [49] studied blockchain security and performance-based PoW. They presented a novel quantitative approach to examine the security and performance implications of various consensus and network parameters applied to PoW blockchains. Therefore, the approach proposed in [49] is solely based on PoW consensus, as opposed to hybrid mechanism approaches that offer higher levels of security and performance. Another study was conducted by [50] to review the role of blockchain in preventing future pandemics. Several applications of blockchain technology were also discussed, and these may assist in fighting the COVID-19 pandemic.
