# A Compute and Wait in PoW (CW-PoW) Consensus Algorithm for Preserving Energy Consumption
## 3 Consensus Protocol Overview  
Bitcoin technology could refer to the most famous blockchain implementation that is created in 2008 by a person or group working under the pseudonym \"[PERSON]\" [9]. In public cryptocurrencies and distributed ledger systems, the fundamental infrastructure of the blockchain is a peer-to-peer overlay network over the Internet [10]. Transactions represent the exchanges between users, and the recorded transactions are grouped together in blocks of size 1M at most. After recording recent transactions, a new block is generated and all transactions will be validated by miners, who will analyze the entire history of the blockchain. If the block is valid, it is time-stamped and integrated into the blockchain. The transactions' contents are then visible on the entire network. Once added to the chain, a block can not be changed or deleted, which guarantees the authenticity and security of the network. Each block in the chain is made up of the following elements: a collection of transactions, the hash (sum of transactions) used as an identifier, the hash of the previous block (except for the first block in the chain, called the genesis block) and the target (a measure of the amount of work that was required to produce the block). The main application of this technology is that of crypto-currencies such as Bitcoin [11]. Beyond its monetary aspect, this decentralized information storage technology could have multiple applications requiring secure exchanges without going through a centralizing body, or unfalsifiable traceability, such as applications based on smart contracts, applications allowing the exchange of all kinds of goods or services, means of improving their predictive systems known as oracles, the traceability of products in the food chain, etc. Each node of the network operates autonomously with respect to the set of rules to which it belongs, and this mechanism of identity management plays a main role in determining the organization of the nodes of a blockchain network. From the system design perspective, a blockchain network contains four levels of implementation. These are the data and network organization protocols, the distributed consensus protocols, the autonomous organization framework based on intelligent contracts and the application (the interface) [12]. In each type of blockchain, several consensus algorithms are designed. One of the most famous algorithms is Proof of Work (PoW), whose concept was first introduced by [PERSON] and [PERSON] in 1993 [13] and in which the authors have presented a computational technique to combat spam in particular and control access to a shared resource in general. The main idea is to require a user to calculate a moderately difficult but not insoluble function, in order to access the resource, thus avoiding frivolous use. Then the work should be difficult to do for the requester, but easily verifiable for a third party. In 1997, [PERSON] implemented the idea with Hashcash, an algorithm to easily produce proofs of work using a hash function (especially SHA-256), and whose main use was electronic mail. The term 'proof of work' has been coined in 1999 by [PERSON] and [PERSON] in their article Proofs of Work and Bread Pudding Protocols [14]. In Bitcoin, to validate a block, the miner had to build a draft of this block (including transactions and payload data), indicate the identifier of the previous block to make the link, and vary a number present in the header called the nonce. By varying this nonce (as well as other parameters in the block), the miner was able to try a gigantic number of possibilities so that the hash of the header produced a suitable result, i.e., a hash starting with a sufficient number of zeroes. Due to the high power consumption of PoW, the Proof of Stake (PoS) is positioned as an alternative. Peercoin was the first cryptocurrencyto use PoS by [PERSON] and [PERSON] in 2012 [15]. PoS asks the user to prove the possession of a certain quantity of cyber money to pretend to validate blocks.