Hash function (hashing)

A cryptographic hash function takes any input, from a single word to a large file, and produces a fixed-length output called a hash or digest. Good hash functions have three key properties: the same input always gives the same output; changing even one character of the input gives a completely different hash; and it is practically impossible to work backwards from a hash to the input or to find two inputs with the same hash.

Blockchains rely on hashing everywhere. Each block contains the hash of the previous block, which links the chain together and makes tampering obvious. Transactions are identified by their hash. Bitcoin addresses are derived from public keys by hashing. And proof-of-work mining is essentially a race to find an input whose hash falls below a target number.

Bitcoin uses SHA-256 for mining and block hashes, while Ethereum uses Keccak-256. Other algorithms such as Scrypt (Litecoin, Dogecoin) were chosen for different mining properties.