Proof of Work

Bitcoin has no CEO, no head office, and no bank checking its transactions. Yet it has kept a reliable record of who owns what since 2009. The system that makes this possible is called proof of work, and it is one of the most important inventions in the history of crypto.

Proof of work turns electricity and computing power into security. Here is how it works and why it still matters.

What Does Proof of Work Mean?

Proof of work (often shortened to PoW) is a consensus mechanism that lets a decentralized network agree on a single history of transactions. Participants called miners bundle pending transactions into a block and then race to solve a mathematical puzzle. The first miner to find a valid solution broadcasts the block to the network, other computers quickly check that the answer is correct, and the block is added to the chain.

Imagine a giant lottery where the only way to get a ticket is to do work. Each ticket costs a little electricity, and nobody can cheat by printing fake ones because every ticket is easy for others to check. Whoever holds the winning ticket gets to write the next page in a shared notebook and collects a prize. The more tickets you buy with work, the better your odds, but nobody can guarantee a win.

How Proof of Work Works

Under the hood, proof of work relies on a special kind of math function called a hash function. A hash function takes any input and produces a fixed-length output that looks random. Changing even one character of the input completely changes the output.

The Mining Puzzle

To create a valid block, miners must find a hash of the block's data that falls below a target number set by the network. The only way to do this is trial and error. Miners repeatedly change a small value in the block called a nonce and hash the block again, sometimes trillions of times per second, until one of the results happens to meet the target.

Difficulty Adjustment

The network automatically adjusts how hard the puzzle is so that blocks arrive at a steady pace. On Bitcoin, the goal is about one block every 10 minutes, and the difficulty is recalculated every 2,016 blocks, which works out to roughly two weeks. If more miners join and blocks come too fast, the puzzle gets harder. If miners leave, it gets easier.

Rewards for Miners

The miner who finds a valid block earns a block reward of newly created coins, plus the transaction fees paid by users in that block. On Bitcoin, the block reward is cut in half roughly every 210,000 blocks, in an event known as the halving. This steadily slows the creation of new bitcoin until the total supply reaches its 21 million limit.

Why Proof of Work Is Secure

Proof of work makes rewriting history extremely expensive. To change a past transaction, an attacker would have to redo the work for that block and every block after it, while also outpacing the honest network as it keeps adding new blocks.

In practice, this would require controlling more than half of the network's total computing power, known as the hash rate. This scenario is called a 51% attack. On a large network like Bitcoin, the cost of the hardware and electricity needed makes such an attack very hard to pull off. Smaller proof of work networks with less computing power have been attacked this way in the past.

Pros and Cons of Proof of Work

Proof of work has a long track record, but it also attracts strong opinions. Here are the main points on each side:

• Battle-tested security: Bitcoin has run on proof of work for well over a decade without its core rules being broken.
• Simple and objective: Anyone can verify which chain has the most accumulated work, without trusting a particular group of people.
• Fair entry: New coins go to whoever contributes work, rather than to people who already hold coins.
• High energy use: Mining consumes a large amount of electricity, which has led to ongoing debate about its environmental impact.
• Hardware concentration: Specialized mining machines and cheap power give large operations an advantage over individual miners.

Proof of Work in Practice

The idea behind proof of work existed before Bitcoin. In 1997, cryptographer Adam Back proposed a system called Hashcash that used a similar puzzle to fight email spam. Satoshi Nakamoto adapted the concept to secure a digital currency, and Bitcoin became the first major blockchain to use it.

Mining Hardware and Pools

Early Bitcoin miners used ordinary computer processors. Over time, mining moved to graphics cards and then to ASICs, which are chips designed to do one job: hash as fast as possible. Because finding a block alone is now very unlikely for a small miner, most miners join mining pools that combine their computing power and share rewards based on each member's contribution. You can learn more about this process in our guide to mining.

Networks That Use Proof of Work

Bitcoin is by far the largest proof of work network. Other well-known examples include Litecoin, Dogecoin, and Monero. Ethereum also used proof of work until September 2022, when it switched to proof of stake in an upgrade known as the Merge.

Useful Links

• What is Bitcoin?
• What is the history of Bitcoin?
• What is blockchain?
• What is crypto mining?