# Hybrid Consensus Algorithm Based on Modified Proof-of-Probability and DPoS
## 2 Background and Related Work
### Proof-of-Work  
The concept of PoW was first proposed by [PERSON] and [PERSON] in their 1993 academic paper [7], while the term proof-of-work was proposed in the 1999 paper [11] published by [PERSON] and [PERSON]. Later, in his 2008 paper [12], [PERSON] took proof-of-work as the consensus algorithm of bitcoin. The success of bitcoin had a great impact on cryptocurrencies, and PoW has become one of the mainstream consensus algorithms.  
The block structure in bitcoin is shown in Figure 1. It consists of version number, previous block hash value, Merkle root, timestamp, difficulty, nonce value, and transaction data. Due to blockchain being a decentralized distributed system, each node has the right to record transactions and package them into blocks. In order to ensure that the blocks are orderly and effective, a method is needed to make each node fairly compete for bookkeeping rights. The algorithm flow is shown in Figure 2. By giving a target hash value, the algorithm requires that the block hash value calculated by the node is less than the target value [13]. The nonce value plays a decisive role in the calculation process. Since the information in the block is known and fixed except for nonce value, we can only guess the nonce value randomly if we want to calculate the block hash value smaller than the target hash value. At the same time, the hash function is irreversible and sensitive to the input value, so no node can cheat. Nodes usually need to go through trillions of attempts to discover the nonce value. The first node to discover the nonce value is regarded as a successful miner, and obtains the cryptocurrency from the system as a reward, and its block will be regarded as effective and published on the blockchain. With the increasing difficulty of mining and the emergence of mining pools, it is almost impossible for individuals with limited resources to succeed in mining. The mining pool concentrates the computing power of multiple nodes to mine and distributes the income to each node according to the computing power proportion after successful mining. Obviously, the emergence of mining pools and mines makes it more difficult for other personal nodes to mine successfully, which obviously goes against the original intention of blockchain. In addition, the computing power of the four largest mining pools once exceeded 50% of the total network, which greatly increased the possibility of a 51% attack [14] and seriously weakened the security of bitcoin.  
Figure 1: Bitcoin block structure.  
Figure 2: Algorithm flow.  
Other problems with bitcoin include resources waste and inefficiency. In order to compete for bookkeeping rights, miners have to keep mining. With the increasing difficulty of mining, the miners will invest more computing resources, which not only wastes a lot of computing resources, but also leads to the increasing power consumption of mining. A recent survey found that bitcoin mining consumes 5 gigawatts of electricity per day. This is slightly less than 1% of the world's total electricity consumption, equivalent to about 4 million households. However, although the price of bitcoin has dropped by about 75%, the computing power of the bitcoin system is rising, which has reached 54 eh/s at present, meaning that the mining cost may be higher than the mining revenue in the future, making the prospect of bitcoin not optimistic. At the same time, in order to prevent hard fork and make block data difficult to be tampered with, bitcoin transaction needs to wait for six blocks to be confirmed before it can be completed. In addition, block capacity, block generation speed, and other factors further limit the transaction speed of bitcoin, which makes it difficult to put bitcoin into practical use on a large scale.