# Hybrid Consensus Algorithm Based on Modified Proof-of-Probability and DPoS
## 5 Evaluation and Results
### Scalability
In the proposed algorithm, due to the combination of two consensus algorithms, the malicious node must have 51% of the computing power and 51% of the token in the system to launch an effective malicious attack, which greatly increases the cost of malicious attacks and improves the security of the system. 2. DDoS attack: In fact, blockchain technology itself can well resist traditional DDoS attacks. This is because the traditional centralized system will absorb a lot of spam information, which may lead to DDoS attacks, and the nature of DDoS attacks makes it almost impossible to achieve the bandwidth needed to process these data. A decentralized blockchain platform allows users to rent their bandwidth, which can be pooled, thus, greatly increasing the amount of data processing and greatly reducing the risk of DDoS attacks. At the same time, for the proposed algorithm, due to the use of the waiting time mechanism and the expected value mechanism, it is easy to verify the validity of the request, so a large number of malicious requests will be easily rejected, and at the same time, force the malicious nodes to enter the waiting time, which significantly reduces the frequency of malicious attacks, and then, enables the blockchain system to effectively resist DDoS attacks. 3. Selfish mining attack: In the general blockchain system, malicious nodes may choose not to disclose their own generated blocks, but to continue mining on their own branch. When the length of the public branch exceeds that of the private branch, the malicious node will publish the private branch to make the mining of the honest node invalid. However, in the proposed algorithm, due to the introduction of the waiting time mechanism and expected value mechanism, block generation only depends on computing power competition to a small extent, and more depends on probability. Therefore, malicious nodes usually do not have the ability to create private branches. In addition, even if a malicious node can generate blocks earlier, it must be disclosed, compared with the expected value, and matched to be considered as a valid block. Therefore, the malicious node can't hide blocks and guarantee their validity. Moreover, it will be regarded as malicious behavior to disclose a blockchain containing several invalid blocks later Then, the malicious node will be removed by voting. Therefore, the proposed algorithm can effectively prevent a selfish mining attack. 4. Sybil attack: In the blockchain system, a single malicious node may disguise as multiple nodes to control the whole network and conduct malicious behavior. In the proposed algorithm, due to the combination of PoP and DPoS, no matter what kind of node the malicious node wants to disguise as, it needs to pay a price. That is, if the malicious node disguises as a PoP node, it must have certain computing power and carry out mining. Only if the node mines successfully can it be regarded as an effective node. If it is disguised as a DPoS node, it must have a certain number of tokens to participate in voting. Obviously, the cost of a Sybil attack by a malicious node is as high as that of 51% attack. Therefore, the proposed algorithm can also better resist Sybil attacks. It is worth mentioning that this algorithm can be further improved to better resist Sybil attacks. For example, adding an authentication mechanism can make use of third-party trusted organizations such as Oracle for authentication, but this will sacrifice the decentralization of the system and the anonymity of nodes. Therefore, a guaranteed algorithm can also be used to ensure that any new node must be guaranteed by the trusted node in advance, which can be used for identity authentication under the premise of ensuring anonymity. To sum up, the comparison between our consensus algorithm and other consensus algorithms is shown in Table 1.