Atbash Cipher
Reverse-alphabet substitution cipher (A↔Z, B↔Y…).
Atbash is a self-inverse cipher (A↔Z, B↔Y, …), so encode and decode produce the same transformation. The toggle exists for UX consistency with the other classical ciphers.
About this tool
Atbash is one of the oldest substitution ciphers — each letter maps to its alphabet mirror. It's its own inverse: encoding twice gets you back the original.