Hash

A hash is the fixed-length output of a mathematical function that turns any input—text, a file, or a block of transactions—into a unique-looking string of characters. The same input always yields the same hash, while even a tiny change produces a completely different result. Hashes are one-way: you cannot practically reverse one to recover the original input.

Blockchains use hashes to link blocks together and to check that data has not been altered. A common misconception is that a hash encrypts or hides data; it does not store the input at all, it only produces a fingerprint of it.