SHA-256

SHA-256 (Secure Hash Algorithm 256-bit) is a cryptographic hash function designed by the US National Security Agency and published by the US National Institute of Standards and Technology (NIST) in 2001 as part of the SHA-2 family. It turns any input into a 256-bit output, usually written as 64 hexadecimal characters.

Bitcoin uses SHA-256 in several places, most famously in mining. Miners hash the block header twice with SHA-256 (often called double SHA-256) and keep changing a number called the nonce until the result is below the network's target. Because there is no shortcut, the only way to find a valid hash is brute-force guessing, which is why Bitcoin mining consumes so much computing power and is dominated by specialised ASIC chips.

SHA-256 is also used widely outside crypto, in TLS website certificates, software signing and password systems. No practical attack that breaks it is publicly known.