Library · Crypto, word by word · Cryptography
What is SHA-256?
SHA-256 is a hash function: it turns any data into a fixed 256-bit fingerprint, usually written as 64 hexadecimal characters. The same input always gives the same fingerprint, any change gives a different one, and the input cannot be worked out from the output. Bitcoin uses it to link blocks and in mining.
What it means for you
Bitcoin block hashes and transaction IDs, the strings you paste into a block explorer, are SHA-256-based. The last word of a seed phrase partly encodes a SHA-256 checksum, so a phrase with one wrong word usually fails a wallet's check instead of quietly opening a different, empty wallet.
How it works
FIPS 180-4 specifies SHA-256 as part of the SHA-2 family. It accepts messages shorter than 2^64 bits, pads them to a multiple of 512 bits, and processes each 512-bit block with 32-bit words and a schedule of sixty-four words, producing a 256-bit digest. NIST calls it secure because finding a message for a given digest, or two messages with the same digest, is computationally infeasible. NIST's post-quantum draft rates its collision security at 128 bits. Bitcoin applies it twice, SHA256(SHA256()), to block headers, transaction IDs and the Merkle root.
Sources: NIST FIPS 180-4: Secure Hash Standard, NIST IR 8547 (initial public draft): Transition to Post-Quantum Cryptography Standards, Bitcoin developer reference: block chain · checked 4 October 2026
Often confused with
On Cryptominium
Related words
Educational content, not financial advice. Written by hand and checked against the source named above. Something wrong? Tell us and we reply within two business days.