How Atbash works
Write the alphabet forwards, then backwards underneath: A sits above Z, B above Y, C above X, and so on to M above N. To use the cipher, swap every letter for the one opposite it. HELLO becomes SVOOL.
Because the swap works the same way in both directions, decoding is exactly the same as encoding. Type or paste any text into the box and the result appears straight away. Capital letters, spaces and punctuation are kept.
Where Atbash comes from
Atbash began as a Hebrew cipher. Its name comes from the first, last, second and second-to-last letters of the Hebrew alphabet: aleph, tav, bet and shin. It appears in the Hebrew Bible, where the Book of Jeremiah uses the name Sheshach for Babel, which is Babel written in Atbash.
There is no key, so Atbash offers no real security: anyone who knows the trick can read it. It is still popular in puzzles, treasure hunts and escape rooms because it is quick to do by hand once you know the pairs.
Atbash is a special kind of substitution cipher, the same family as cryptograms. If a puzzle uses a mixed-up alphabet instead of a reversed one, the cryptogram solver can crack it.
To decode Atbash by hand, it helps to learn the pairs in two halves: A to M on top and Z to N underneath. A and Z, B and Y, C and X, D and W, E and V are the pairs you will use most often.
Questions and answers
How do I decode an Atbash message?
Paste it into the box. Atbash decodes exactly the same way it encodes, so the plain message appears straight away.
Does Atbash use a key?
No. The alphabet is always reversed in the same way, which is why it is easy to break.
What happens to numbers?
Numbers and punctuation are left unchanged.
What is the difference between Atbash and ROT13?
ROT13 shifts every letter 13 places along the alphabet, while Atbash reverses the alphabet. Both undo themselves when applied twice.