Hash

A hash function takes any input, from a single word to a gigabyte file, and produces a short fixed-length output called a hash or digest. The same input always gives the same hash, but changing even one character gives a completely different result, and there is no practical way to work backward from the hash to the input. Bitcoin uses SHA-256, which produces a 256-bit value usually shown as 64 hexadecimal characters.

Blockchains use hashes everywhere. Each block includes the hash of the previous block, which is what chains them together. Transactions are identified by their hash, so a transaction ID like 3a7b... is just the hash of the transaction data. Addresses are derived by hashing public keys.

In proof-of-work mining, miners repeatedly hash a block header with different values until they find a hash below a target number. That search is what hash rate measures. Hashes are also used to verify that downloaded software has not been altered, by comparing the file's hash with one published by the developers.