Proof of work

General introduction

Blockchain technology is based on the use of consensus mechanisms. These mechanisms are intended to verify and secure the authenticity of the data in order to eliminate the need for control intermediaries. Bitcoin, the first of the crypto-currencies, was based on proof of work (or proof of work) in order to allow individuals to carry out exchanges without having to resort to a bank.

This consensus mechanism is the most reliable mechanism today because it secures a blockchain with pure computing power, so that no trust is required in the validating nodes. We therefore measure the level of security of a blockchain based on proof of work by the computing power allocated to it (hashrate = hashes per second). That of Bitcoin recently reached 171 exahashes per second. Such computing power provides extremely important security, as evidenced by the Bitcoin blockchain, which has never been compromised in ten years.

The characteristics of this mechanism


This consensus mechanism establishes a principle of competition between the validating nodes called “miners”. Miners are people who own computer hardware dedicated to solving the proof-of-work mechanism of a blockchain. A miner has mined a block when he has managed to validate all the transactions it contains. The solution found is then submitted to the other miners, who then check that the proposed block is correct. Here we can take the example of Sodoku. The miners each try to be the first to solve a grid (of particularly high complexity). The first to find the solution presents it to the other miners, who will ensure that the completed grid does not contain any errors. This image helps to understand that it is difficult to mine a new block but very easy to check if the mined block is correct.


We talk about competition, because only the first miner to find the new block is rewarded. It is important to note that the more miners there are, the higher the complexity of the cryptographic puzzle (to continue with the aforementioned example, we can say that the grid becomes larger or smaller with more or fewer initial digits). The blockchain self-regulates the difficulty with regard to the power allocated to it. This function ensures that, regardless of the number of miners, a new block is always mined within a defined time (at 10-minute intervals for Bitcoin)


It is crucial to look at the computing power allocated to blockchains that rely on the proof-of-work consensus mechanism because relatively low power can be a source of vulnerability.

Let’s take the example of a 51% attack, the most commonly mentioned threat against proof of work. During this operation, the attackers will try to gather a computing power at least equivalent to 51% of the total power attributed to the network (Bitcoin: 51% of 100 exahashes = 51 exahashes). The higher the computing power attributed to a blockchain, the more the attacker will have to dedicate computer hardware and electricity to it to achieve his ends. If successful, the other miners on the network are neutralized because they are unable to compete with the attacker's computing power. The latter can then take control of the blockchain and produce its own blocks or modify the content of blocks already mined. Note that this operation requires maintaining a computing power of 51%; in the event of a loss of dominance, the attacker no longer has control over the blockchain.


The electricity consumption of this consensus mechanism is a regularly raised issue within the community. The competitive aspect that provides this level of security leads to a fairly significant waste of energy. The electricity used by miners who fail to mine a new block is indeed lost: with each block, the whole process is started again from scratch by all the miners. Other consensus mechanisms, such as proof of stake or authority, are much more economical on this point, but do not provide the same level of security and decentralization.

However, it is important to take into account that miners always seek to be as profitable as possible. They are therefore placed near the places of energy production in order to be able to recover the electricity that the network is not able to transport at an advantageous price. The electricity that was supposed to be lost is therefore finally used.

Finally, the proof of work has a derivative mechanism: the proof of work with delay (delayed proof of work). It allows you to rely on the computing power of a blockchain that uses proof of work to secure your own blockchain. The electricity consumed by a blockchain then benefits the security of several of them.

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