Atbash Cipher Encoder and Decoder
Flip the alphabet so A becomes Z, B becomes Y, and so on. Atbash undoes itself, so the same box encodes and decodes.
How the Atbash cipher works
Atbash writes the alphabet backwards and swaps each letter for the one in the same spot. A becomes Z, B becomes Y, C becomes X, and M and N trade places in the middle. There is no key and nothing to set. Because every pair just swaps, running Atbash a second time puts every letter back. So one box does both jobs: paste plain text to encode it, or paste Atbash to decode it.
Only A to Z change, and capitals stay capitals. Numbers, spaces, punctuation and accented letters come through as they are. It all runs in your browser.
A worked example
Take HELLO WORLD. H is the 8th letter, so it swaps with the 8th letter from the end, S. E swaps with V, L with O, O with L, W with D, R with I, and D with W. That gives SVOOL DLIOW. Put SVOOL DLIOW back in and you get HELLO WORLD. Some words land on other real words: WIZARD comes out as DRAZIW, which is WIZARD backwards. That only happens because W and D, I and R, Z and A are all mirror pairs.
Where the name comes from
Atbash started with the Hebrew alphabet. The name is built from the letters it swaps: alef (the first letter) with tav (the last), then bet (the second) with shin (the second to last). A-t-b-sh. The Faculty of Theology and Religion at Oxfordpoints to two places it shows up in the Book of Jeremiah. In Jeremiah 25:26, “Sheshach” is Atbash for Babel, meaning Babylon. In Jeremiah 51:1, “Leb-qamai” decodes to Kasdim, the Chaldeans.
Is Atbash secure?
No. With no key, anyone who knows the trick can read it, and anyone who doesn’t can guess it from a short sample. Use it for puzzles, games and notes to a kid who likes codes. If you want a cipher you can change, try the Caesar cipher (pick any shift) or the Vigenère cipher (pick a keyword). For anything private, use Encrypt Text. The types of ciphers guide explains how they all fit together.