About this tool
The Atbash Cipher Encoder / Decoder is a free, in-browser tool for an ancient substitution cipher that originated with the Hebrew alphabet. It replaces every letter with its mirror image across the alphabet: A becomes Z, B becomes Y, C becomes X, and so on. For an uppercase character the output is chr(90 − (code − 65)); for lowercase it is chr(122 − (code − 97)). In other words the letter at index i (0–25) maps to the letter at index 25 − i.
Atbash uses no key at all, and it is an involution — applying it twice returns the original text — so the very same operation both encodes and decodes. Just type or paste your message and read the result. All processing happens locally in your browser; nothing you enter is sent anywhere.
Case is preserved and non-letters such as spaces, digits, and punctuation pass through untouched. Atbash appears throughout puzzle culture, from Bible study and the Dead Sea Scrolls to modern escape rooms and the lore of The Da Vinci Code, and it is a common first step when identifying an unknown cipher. Because it is fixed, it offers no real security — it is a puzzle and teaching cipher rather than a way to protect secrets.