# Assessing Blockchain Consensus and Security Mechanisms against the 51% Attack
## 7 Analysis  
In this section, we analyze the effectiveness of five security protection techniques against the 51% attack. Some of them are research prototype, and yet to be implemented on a real system. Figure 6 presents an evaluation framework, where we classify the risk elements of each security technique as low, medium or high. The security-based framework also presents the advantage, vulnerability, cost, and functionality of the mitigation techniques. The penalty system for delayed block submission is a PoW-based security technique and yet to be implemented in the real network. This technique is a research prototype and includes various limitations. Based on the proposal, a block delay will be imposed over the privately mined chain. Assuming an adversary aims to beat the six confirmation times, the introduced block delay will be only 21 blocks. As a result, the attacker is forced to mine another 21 blocks in a sequence to lift the delay and get his mined blocks to be included in the normal chain. According to [PERSON], in any occasion, when an adversary owns 51% of the network hash, they will always succeed regardless of the enforced delay [12]. Therefore, the possibility to perform the 51% attack when this security mechanism is in place is very big, especially when targeting coins that have very low network hashing power. In addition, the delay process slows the overall transactions of the network and intensely impacts the regular transactions. Transactions that are added to the delay blocks will not be confirmed until the penalty is lifted. Thus, it may take a few hours or even a few days to confirm some transactions. Hence, this technique is not very appropriate to be implemented in a real network, it is not fully effective against 51% attack, and it has a medium level risk. Komodo blockchain appoints notary nodes for notarization. The nodes are appointed to retrieve the information from Komodo and save it in the bitcoin blockchain. In order to carry out double-spending, an attacker needs to rewrite the Komodo chain and bitcoin checkpoints. The attacker also needs to influence the majority of the notary network [48]. Hence, the whole process makes the technique quite robust. However, a limited number of nodes makes this security technique centralized, which introduces the widely known issue of a \"single point of failure\", where attackers know exactly what to attack. The dPoW is not cost-effective as it requires an implementation fee to be implemented  
Figure 6: A security-based framework of defense mechanisms based on the 51% attack. to the blockchain. It can only be implemented to coins that are based on UTXO-based blockchain. Moreover, participants are required to wait for an explicit amount of time for the notarization process to be completed. It may discourage particular participants who aim to have a faster transaction. The notarization process is done every 10 min. This 10 min checkpoint time gives attackers a vast amount of time to conduct the 51% attack on crypto-coins in which the block confirmation time needs just a few seconds [48]. The attack risk of this security technique is low. PirlGuard was developed to provide protection against 51% exploitation. This security technique is based on only the Ethash algorithm. It follows a similar approach to the \"The Penalty System for Delayed Block Submission\", which introduces a block delay. As discussed, the penalty system is vulnerable to the 51% attack; therefore, it is relatively easy for an attacker to achieve the required blocks. PirlGuard also involves master nodes for notarization, a centralized feature that makes the network weak because of the \"single point of failure\" security issue. PilGuard establishes that it mitigates the 51% attack to approximately 0.03%. Therefore, both the penalty system and master node feature have a low risk level. ChainLocks also aims to provide protection against the 51% attack. The major limitation of ChainLocks is that it only protects one currency.