What is a hash?
In one sentence: A fixed-length fingerprint of some data. One-way: you can't get the data back from it.
A little more
A hash function turns any data (a word, a photo, a whole film) into a short fixed-length code. The same data always gives the same code, but change a single letter and the code changes completely. You can't run it backwards to get the data out.
Hashes check that downloads aren't damaged, spot duplicate files, and, when made deliberately slow and salted, let websites check passwords without storing them.
It's a bit like… A fingerprint: it identifies a person perfectly, but you can't rebuild the person from it.
See it in action
- 4Secret codes: encryption and hashingFrom Caesar's cipher to the AES-256 lock behind every padlock: encrypt and decrypt in your browser, crack a code by brute force, and watch one letter change a SHA-256 fingerprint completely..7 min · interactive
- 15How does a website check your password without keeping it?Make up a password, store it on three pretend websites (plain text, a plain fingerprint, salted and slow), then let a thief steal all three.8 min · interactive