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Proof-of-work (PoW)

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Ethereum, like Bitcoin, currently uses a consensus protocol called Proof-of-work (PoW). This allows the nodes of the Ethereum network to agree on the state of all information recorded on the Ethereum blockchain and prevents certain kinds of economic attacks.

Over the next year, proof-of-work will be phased out in favour of Proof-of-stake (PoS). The transition to proof-of-stake will also phase out mining from Ethereum. For more details on timing, check the progress of the Eth2 merge upgrade.

Prerequisites

To better understand this page, we recommend you first read up on transactions, blocks, and consensus mechanisms.

What is Proof-of-work (PoW)?

Proof-of-work is the mechanism that allows the decentralized Ethereum network to come to consensus, or agree on things like account balances and the order of transactions. This prevents users from "double spending" their coins and ensures that the Ethereum chain is tremendously difficult to attack or manipulate.

Proof-of-work and mining

Proof-of-work is the underlying algorithm that sets the difficulty and rules for the work miners do. Mining is the "work" itself. It's the act of adding valid blocks to the chain. This is important because the chain's length helps the network follow the correct Ethereum chain and understand Ethereum's current state. The more "work" done, the longer the chain, and the higher the block number, the more certain the network can be of the current state of things.

More on mining

How does Ethereum's proof-of-work work?

Ethereum transactions are processed into blocks. Each block has a:

  • block difficulty – for example: 3,324,092,183,262,715
  • mixHash – for example: 0x44bca881b07a6a09f83b130798072441705d9a665c5ac8bdf2f39a3cdf3bee29
  • nonce – for example: 0xd3ee432b4fb3d26b

This block data is directly related to proof-of-work.

The work in proof-of-work

The proof-of-work protocol, Ethash, requires miners to go through an intense race of trial and error to find the nonce for a block. Only blocks with a valid nonce can be added to the chain.

When racing to create a block, a miner will repeatedly put a dataset, that you can only get from downloading and running the full chain (as a miner does), through a mathematical function. The dataset gets used to generate a mixHash below a target nonce, as dictated by the block difficulty. The best way to do this is through trial and error.

The difficulty determines the target for the hash. The lower the target, the smaller the set of valid hashes. Once generated, this is incredibly easy for other miners and clients to verify. Even if one transaction were to change, the hash would be completely different, signalling fraud.

Hashing makes fraud easy to spot. But proof-of-work as a process is also a big deterrent to attacking the chain.

Proof-of-work and security

Miners are incentivised to do this work on the main Ethereum chain. There is little incentive for a subset of miners to start their own chain – it undermines the system. Blockchains rely on having a single state as a source of truth. And users will always choose the longest or "heaviest" chain.

The objective of proof-of-work is to extend the chain. The longest chain is most believable as the valid one because it's had the most computational work done. Within Ethereum's PoW system, it's nearly impossible to create new blocks that erase transactions, create fake ones, or maintain a second chain. That's because a malicious miner would need to always solve the block nonce faster than everyone else.

To consistently create malicious yet valid blocks, you'd need over 51% of the network mining power to beat everyone else. You'd need a lot of computing power to be able to do this amount of "work". And the energy spent might even outweigh the gains you'd make in an attack.

Proof-of-work economics

Proof-of-work is also responsible for issuing new currency into the system and incentivizing miners to do the work.

Miners who successfully create a block get rewarded with two freshly minted ETH and all the transaction fees within the block. A miner may also get 1.75 ETH for an uncle block. Uncle blocks are valid blocks created by a miner practically at the same time as another miner mined the successful block. Uncle blocks usually happen due to network latency.

Finality

A transaction has "finality" on Ethereum when it's part of a block that can't change.

Because miners work in a decentralized way, two valid blocks can get mined at the same time. This creates a temporary fork. Eventually, one of these chains will become the accepted chained after a subsequent block has been mined and added, making it longer.

But to complicate things further, transactions rejected on the temporary fork may have been included in the accepted chain. This means it could get reversed. So finality refers to the time you should wait before considering a transaction irreversible. For Ethereum, the recommended time is six blocks or just over 1 minute. After six blocks, you can say with relative confidence that the transaction was successful. You can wait longer for even greater assurances.

Finality is something to bear in mind when designing dapps. It would be a poor user experience to misrepresent transaction information to your users, especially if the transaction is of high value.

Remember, this timing doesn't include the wait times for having a transaction picked up by a miner.

Proof-of-work energy-usage

A major criticism of proof-of-work is the amount of energy output required to keep the network safe. To maintain security and decentralization, Ethereum on proof-of-work consumes 73.2 TWh annually, the energy equivalent of a medium-sized country like Austria.

Pros and cons

ProsCons
Proof-of-work is neutral. You don't need ETH to get started and block rewards allow you to go from 0ETH to a positive balance. With proof-of-stake you need ETH to start with.Proof-of-work uses up so much energy that it's bad for the environment.
Proof-of-work is a tried and tested consensus mechanism that has kept Bitcoin and Ethereum secure and decentralized for many years.If you want to mine, you need such specialized equipment that it's a big investment to start.
Compared to proof-of-stake it's relatively easy to implement.Due to increasing computation needed, mining pools could potentially dominate the mining game, leading to centralization and security risks.

Compared to proof of stake

At a high level, proof-of-stake has the same end goal as proof-of-work: to help the decentralized network reach consensus securely. But it has some differences in process and personnel:

  • Proof-of-stake switches out the importance of computational power for staked ETH.
  • Proof-of-stake replaces miners with validators. Validators stake their ETH to activate the ability to create new blocks.
  • Validators don't compete to create blocks, instead they are chosen at random by an algorithm.
  • Finality is clearer: at certain checkpoints, if 2/3 validators agree on the state of the block it is considered final. Validators must bet their entire stake on this, so if they try to collude down the line, they'll lose their entire stake.

More on proof-of-stake

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