The hash

A fingerprint of fixed size#

A hash function turns any data, a word or a whole block, into a number of fixed size. Bitcoin uses SHA-256, whose result is 256 bits, written as 64 hexadecimal characters (0 to 9, a to f). Three properties matter:

• Deterministic: the same data always gives the same hash, on any computer.
• One-way: from a hash, there is no way back to the data, and no way to aim for a chosen result.
• Unpredictable: the smallest change produces an unrelated hash.

Change one character, change everything#

Edit the text below. The hash is computed in your browser at each keystroke; the characters that changed since your last edit are highlighted. Changing a single letter typically changes about 60 of the 64.

This is what makes mining a pure game of trial and error: you cannot nudge a block toward a good hash, you can only try another variant and look at the result.

Bitcoin hashes the header, twice#

Miners do not hash the whole block, only its 80-byte header, and they apply SHA-256 twice in a row (SHA-256d). The transactions are still covered: the header contains their Merkle root, so changing any transaction changes the hash of the block.

In Mini Miner#