## 2 Overview of Blockchain Technology
### Blockchain Consensus Algorithms
#### 2.2.1 Proof Based Consensus  
The element that yields convincing evidence receives the potential to introduce a fresh block to the blockchain and obtain compensation in proof-based consensus. One option, among the inaugural consensus techniques for blockchain consensus that were originally established, is Proof of Work (PoW) [96]. In PoW, a node is required to prove its trustworthiness by performing work (mining). Specifically, a hard problem to solve, such as guessing the nonce value when the hash code of the nonce and block is required to be matched with the challenge level (bits). Thus, the first node that spends computational resources to figure out the problem obtains the opportunity to insert the block after verifying and confirming every operation in the freshly created block. However, due to energy expenditure, PoW is known to be less energy efficient.  
In Proof of Stake (PoS) [97], the element that shows the highest ownership of the currency becomes the winner, assuming richer nodes have a low probability of attacking the blockchain. Once per time frame, a miner is picked at random, depending on their stake, to contribute a block; thus, PoS is an energy-efficient approach compared to PoW. However, PoS has some drawbacks, such as the dominance of wealthier nodes, its attack-prone nature due to mining costs being much lower, the nothing-at-stake problem, etc.  
Since both PoW and PoS have their own drawbacks, a hybrid approach combining the features of both PoW and PoS has been proposed known as Proof of Activity (PoA) [98], where a group of validators have to digitally sign a produced block for it to be considered legitimate, such that, even if there is a dominant node with a high stake, it cannot control block creation on its own. In PoA, miners first engage in PoW to create new blocks, while PoS is applied to generate subsequent blocks based on the stake. As a drawback, PoA consumes considerable time for the transition from PoW to PoS.  
Proof of Space (PoSp) [99] involves nodes allocating a significant amount of storage for storing predefined data (plots). In PoSp, the node that has the highest plot space obtains the opportunity to synthesize a block. Even though PoSp uses less energy to operate compared to PoW, it can be attacked by storage pre-computing.  
In Proof of Burn (PoB) [100], the miner who burns the highest amount of cryptocurrency token within a given amount of time obtains the chance to mine a new block. PoB involves miners voluntarily destroying their own cryptocurrency by sending it to a burning address, assuming that legitimate users burn more cryptocurrency. However, PoB may discourage users as it involves the permanent burning of cryptocurrency tokens.  
In Proof of Authority (PoAu) [101], authority is assigned to a particular set of pre-approved validators, typically chosen based on reputation, who are allowed to verify and append fresh blocks. But PoAu has some degree of centralized authority, which reduces the distributed consensus features of the blockchain.  
Proof of Elapsed Time (PoET) [102] is a novel consensus procedure in which nodes ask a trustworthy entity to wait (for an elapsed time), and the element with the shortest waiting time is given the chance to process the subsequent block. However, PoET has been known to suffer from scalability issues, and the wait time determined by execution environment may not be accessible to every node in a decentralized manner.