Atbash cipher

Type or paste on the left. The mirrored text appears on the right, and the same step reverses it, so the swap button is switched off.

There's nothing to configure, because Atbash has no key. Capitals and punctuation stay put, so the shape of the message survives even though the letters don't. If you aren't sure the message is Atbash at all, the panel below checks it against everything else we recognise.

Write the alphabet forwards, then write it backwards underneath. A pairs with Z, B with Y, M with N. There's no key to remember.

The mirror is also its weakness. Letter frequencies come through untouched, so whatever stands in for E in a long message is still the commonest letter on the page, and the rest unravels from there.

Atbash started in Hebrew, and the name is the instruction. Aleph goes to tav, beth to shin, first letter to last, second to second-last.

It appears in the Book of Jeremiah, where Sheshach stands in for Babel. At 51:41 Babylon is named in the parallel clause, so the substitution is unmistakable, and any reader who knew the trick would have seen Babel immediately.

On the Latin alphabet it survives as a puzzle. Escape rooms use it, and so does the occasional geocache.

ROT13 rotates the alphabet where Atbash reverses it, and its shift control covers every Caesar cipher from 1 to 25.

Handed a message with no idea what was done to it? The cipher identifier tries Atbash, the shifts and the text encodings together and ranks what comes back.