Hash function (SHA-256)

A hash function takes data of any length and returns a fingerprint of fixed length. The same input always gives the same output, a 1-bit change gives an unrelated output, and there is no practical way to work backwards from the fingerprint to the data. Hashing is not encryption. There is no key and nothing to decrypt.

Bitcoin uses SHA-256, which produces 256-bit fingerprints, almost everywhere. Block headers are hashed twice for proof of work, transactions are identified by their hash, Merkle trees commit to a block’s contents, and addresses derive from hashed keys. Mining ASICs are machines built to compute SHA-256 and nothing else.