Atbash cipher
Atbash cipher. A classical cipher, computed on your device.
Runs on your device. The file is never uploaded.
Atbash cipher maps every letter of your text to the one at position 25 minus its own, so a becomes z and m becomes n. Uppercase input comes back uppercase, and anything outside the 26 English letters is left alone. The mapping is fixed and carries no key, so recognising it is the whole of reading it.
Questions
What does Atbash do?
It mirrors the alphabet. a becomes z, b becomes y and so on, mapping each letter at position i to position 25 minus i. There is no key and no options at all. It comes from Hebrew scribal practice and turns up mostly in puzzles and CTF challenges now.
How do I decode Atbash text?
Run this same tool on it. The mapping is its own inverse, since mirroring a mirrored alphabet returns the original, so applying it twice gives back exactly what you started with, case and punctuation included. There is no separate decode tool and nothing to configure in either direction, so there is no setting that can be mismatched.
Is it secure?
No. There is no key, so anyone who recognises the cipher reads the text immediately. It is a puzzle, in the same class as ROT13. For anything you need kept private, use AES-GCM encrypt, which is a real cipher with a password you choose.
Can I do the same thing with the substitution tool?
Yes. Substitution cipher encode and Substitution cipher decode both default to the key zyxwvutsrqponmlkjihgfedcba, which is the reversed alphabet and produces exactly this mapping. This tool is that same transformation with nothing to configure and nothing to mistype, which is why it exists separately. Use the substitution tools when you want any other 26-letter permutation.
What about case, digits and punctuation?
Case is preserved, so an uppercase letter mirrors to an uppercase letter. Digits, spaces and punctuation pass through untouched, since only the 26 English letters are mapped, and accented and non-Latin letters are left alone as well. Word lengths and spacing therefore survive the transformation, which is part of why the cipher is recognisable on sight.
Does the text leave my browser?
No. The mapping runs in a Web Worker in this tab over the file you dropped, and the result is shown on the page. Nothing is uploaded and nothing is saved between runs. The site keeps working with the network off, so the text stays on your machine even though the cipher hiding it protects nothing.