Proof-of-work is the decentralized consensus mechanism that underpins public blockchain networks like Bitcoin and Ethereum. It allows for all participants in the network to agree on the validity of transactions and maintain an immutable record of those transactions without relying on a centralized authority.

The core idea behind proof-of-work is that participants in the network, called miners, must expend computing power to find a solution to a complex cryptographic puzzle. This puzzle requires miners to vary a piece of data called a “nonce” until the cryptographic hash of the block header results in a value lower than the current network difficulty target. Finding this proof-of-work requires a massive amount of computing power and attempts. Only when a miner finds a valid solution can they propose the next block to be added to the blockchain and claim the block reward.

By requiring miners to expend resources (electricity and specialized computer hardware) to participate in consensus, proof-of-work achieves several important properties. First, it prevents Sybil attacks where a single malicious actor could take over the network by creating multiple fake nodes. Obtaining a 51% hashrate on a proof-of-work blockchain requires an enormous amount of specialized mining equipment, making these attacks prohibitively expensive.

Second, it provides a decentralized and random mechanism for selecting which miner gets to propose the next block. Whoever finds the proof-of-work first gets to build the next block and claim rewards. This randomness helps ensure no single entity can control block production. Third, it allows nodes in the network to easily verify the proof-of-work without needing to do the complex calculation themselves. Verifying a block only requires checking the hash is below the target.

The amount of computing power needed to find a proof-of-work and add a new block to the blockchain translates directly to security for the network. As more mining power (known as hashrate) is directed at a blockchain, it becomes exponentially more difficult and expensive to conduct a 51% attack. Both the Bitcoin and Ethereum networks now have more computing power directed at them than most supercomputers, providing immense security through their accumulated proof-of-work.

For a blockchain following the proof-of-work mechanism, the rate at which new blocks can be added is limited by the difficulty adjustment algorithm. This algorithm aims to keep the average block generation time around a target value (e.g. 10 minutes for Bitcoin) by adjusting the difficulty up or down based on the hashrate present on the network. If too much new mining power joins and blocks are being found too quickly, the difficulty will increase to slow block times back to the target rate.

Likewise, if older mining hardware is removed from the network causing block times to slow, the difficulty is decreased to regain the target block time. This dynamic difficulty adjustment helps a proof-of-work blockchain maintain decentralized consensus even as exponential amounts of computing power are directed towards mining over time. It ensures the block generation rate remains stable despite massive changes in overall hashrate.

While proof-of-work secures blockchains through resource expenditure, it is also criticized for its massive energy consumption as the total hashrate dedicated to chains like Bitcoin and Ethereum continues to grow. Estimates suggest the Bitcoin network alone now consumes around 91 terawatt-hours of electricity per year, more than some medium-sized countries. This environmental impact has led researchers and other blockchain communities to explore alternative consensus mechanisms that aim to achieve security without high computational resource usage like proof-of-stake.

Nonetheless, proof-of-work has remained the primary choice for securing public blockchains since it was introduced in the original Bitcoin whitepaper. Over a decade since Bitcoin’s inception, no blockchain at scale has been proven secure without either proof-of-work or a hybrid consensus model. The combinations of randomness, difficulty adjustment, and resource expenditure provide an effective, if energy-intensive, method for distributed ledgers to reach consensus in an open and decentralized manner without a centralized operator. For many, the trade-offs in security and decentralization are worthwhile given present technological limitations.

Proof-of-work leverages economic incentives and massive resource expenditure to randomly select miners to propose and verify new blocks in a public blockchain. By requiring miners to find solutions to complex cryptographic puzzles, it provides crucial security properties for open networks like resistance to Sybil attacks and a random/decentralized consensus mechanism. This comes at the cost of high energy usage, but no superior alternative has been proven at scale yet for public, permissionless blockchains. For its groundbreaking introduction of a working decentralized consensus algorithm, proof-of-work remains the preeminent choice today despite improvements being explored.