# Distributed Ledger Technology Review and Decentralized Applications Development Guidelines
## 2 Protocol and Network Tier
### Network Layer
#### 2.2.3 Consensus Protocols
Such implementations aim to do research work (or Proof-of-Research). They gather the computational power across the network to provide solutions to some of the world's problems. CureCoin [89] is implementing an algorithm called SigmaX that aims to perform protein unfolding to find a cure for different diseases. Proof of Activity [90] is a PoW alteration algorithm found in Decred [91]. The algorithm starts as a simple PoW algorithm until one correct hash is found; the block is then transmitted in the network, but it is not yet added to the blockchain. To become a valid block, it needs to be signed by N holders in the network. The PoW obtained hash is used to generate N numbers that correspond to N coins generated since the genesis of the blockchain. Each of these coinshas one current stakeholder who will be required to sign the current block. The signature of all the N stakeholders is required to consider the block valid. In case that some of the stakeholders are not online and cannot sign, then the miners will continue their job to find a new hash and ask other stakeholders to sign the block. This approach makes attacks upon the network more difficult since it makes use of the advantages brought by both systems. _Virtual Mining Protocols_ offer an alternative to the PoW by keeping a high cost for the Prover, but changing the resource consumed. If the cost of the Prover in PoW is the energy consumed, which would be lost if the Prover does not offer honest work to be validated and rewarded by the network, in the virtual mining Protocols the cost is a deposit of coins that are offered as insurance for their honest work. If up until now the node was chosen based on its result to the computationally intensive problem, now the node will be elected in a pseudo-random way, and the chance of winning will be proportional to the number of coins/stakes of the owner of the system. Thus, in Virtual Mining Protocols, the clients have the mining potential proportional to the percentage of the stake they hold. Four virtual mining approaches have been identified across different solutions: Proof of Stake considers the age of the coin in the algorithm, thus requiring for some coins not to be spent for some time; Proof of Burn requires a relevant amount of coins to be destroyed and a proof of the destroying transaction to be provided; Proof of Deposit requires for some coins to be put away for some time in a vault; Proof of Authority suggests that only trusted parties are entitled to provide commits to the system, which can be required where high-security properties need to be implemented [92], like in the case of private Enterprise solutions. However, all four algorithms have the same purpose that is, incentivizing the honest work of the miner by promising as a reward a sum of coins greater than the initial insurance. According to [93], the Casper version of Proof-of-Stake (PoS) is considered a suitable alternative for the permissioned systems, by considering only a fixed set of users as validators of blocks. Another flavor of Proof-of-Stake commonly used for permissioned systems is the Delegated Proof of Stake (DPoS). In DPoS, N witnesses are periodically selected by stakeholders of the system, such that enough decentralization is ensured. Out of the N witnesses, each witness has its chance to propose the next block, and then be rewarded for its contribution. From existing Virtual Mining Protocols, the PoS a good potential of becoming the most used consensus protocol in DLTs because it addresses fundamental problems of the PoW protocol such as computational waste and high-power demand [94]. Anyway, in the case of the PoS algorithm, since the nodes propose a new block by guaranteeing with their stake it gives rise to the \"nothing-at-stake\" vulnerability. This means that when a fork appears in the context of a network partitioning, an attacker node can propose a block on either chain, hoping that at least one block will be accepted.