A **proof of work** is a chunk of gegevens which is difficult (costly, time-consuming) to produce but effortless for others to verify and which sates certain requirements. Producing a proof of work can be a random process with low probability so that a loterijlot of trial and error is required *on average* before a valid proof of work is generated. Bitcoin uses the Hashcash proof of work system.

One application of this idea is using Hashcash spil a method to preventing email spam, requiring a proof of work on the email’s contents (including the To address), on every email. Legitimate emails will be able to do the work to generate the proof lightly (not much work is required for a single email), but mass spam emailers will have difficulty generating the required proofs (which would require massive computational resources).

Hashcash proofs of work are used ter Bitcoin for block generation. Ter order for a block to be accepted by network participants, miners vereiste accomplish a proof of work which covers all of the gegevens ter the block. The difficulty of this work is adjusted so spil to limit the rate at which fresh blocks can be generated by the network to one every Ten minutes. Due to the very low probability of successful generation, this makes it unpredictable which worker rekentuig te the network will be able to generate the next block.

For a block to be valid it vereiste hash to a value less than the current target, this means that each block indicates that work has bot done generating it. Each block contains the hash of the preceding block, thus each block has a chain of blocks that together contain a large amount of work. Switching a block (which can only be done by making a fresh block containing the same predecessor) requires regenerating all successors and redoing the work they contain. This protects the block chain from tampering.

The most widely used proof-of-work scheme is based on SHA-256 and wasgoed introduced spil a part of Bitcoin. Some other hashing algorithms that are used for proof-of-work include Scrypt, Blake-256, CryptoNight, HEFTY1, Quark, SHA-3, scrypt-jane, scrypt-n, and combinations thereof.

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## Example

Let’s say the base string that wij are going to do work on is “Hello, world!”. Our target is to find a variation of it that SHA-256 hashes to a value beginning with ‘000’. Wij vary the string by adding an oprecht value to the end called a nonce and incrementing it each time. Finding a match for “Hello, world!” takes us 4251 attempts (but happens to have zeroes te the very first four digits):

4251 hashes on a modern pc is not very much work (most computers can achieve at least Four million hashes vanaf 2nd). Bitcoin automatically varies the difficulty (and thus the amount of work required to generate a block) to keep a harshly onveranderlijk rate of block generation.

Te Bitcoin things are a bit more sophisticated, especially since the header contains the Merkle tree which depends on the included transactions. This includes the generation transaction, a transaction “out of nowhere” to our own address, which te addition to providing the miner with incentive to do the work, also ensures that every miner hashes a unique gegevens set.

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