SHA-256 stands for Secure Hash Algorithm, 256-bit. It belongs to the SHA-2 family, designed by the US National Security Agency and published by NIST in 2001. It is a public standard used everywhere, from website certificates to software downloads.
What it does
SHA-256 turns any input into a 256-bit hash. There are 2256 (about 1077) possible outputs, only a few orders of magnitude below the estimated number of atoms in the observable universe. Guessing an input that produces a specific output is hopeless.
How Bitcoin uses it
Double hashing. Bitcoin usually applies SHA-256 twice in a row (SHA-256d) to block headers and transaction IDs. This protects against length-extension attacks, a known weakness of single SHA-256 in some uses.
Mining. Mining means hashing an 80-byte block header with SHA-256d again and again, changing a value called the nonce, until the result is below the target. That is the core of Bitcoin's proof of work.
Addresses. Older address types hash a public key with SHA-256 and then RIPEMD-160 to make a shorter identifier.
Why special hardware?
Because mining is nothing but SHA-256 repeated trillions of times, engineers built chips that do only that. These ASIC miners are millions of times more efficient than a normal computer. Combined, the whole network performed more than a sextillion (1021) hashes per second as of October 2026.
Is it safe?
No practical attack on SHA-256 is known after more than two decades of public study. Quantum computers are sometimes mentioned, but known quantum algorithms only give a square-root speedup against hashing, (Grover's algorithm), so SHA-256 would keep roughly 128-bit security against preimage attacks. That is still far beyond reach. The bigger long-term quantum question concerns signatures, not hashing.