ROT13 Cipher
Encrypt and decrypt with ROT13 ("rotate by 13 places"), a simple substitution cipher that replaces a letter with the 13th letter after it in the alphabet. Because the Latin alphabet has 26 letters, ROT13 is its own inverse: applying it twice returns the original text. It is historically used in online forums and Usenet to hide spoilers or punchlines. Everything runs locally in your browser.
Encrypt (plaintext → cipher)
Decrypt (cipher → plaintext)
The math. ROT13 is a special case of the Caesar cipher where the shift is exactly 13. For any letter $L$ at position $i$ (0-25), the transform is $S = (i + 13) \pmod{26}$. Because $13 + 13 = 26 \equiv 0 \pmod{26}$, applying the operation twice restores the original letter. Pairs with the Caesar, Vigenère, and Atbash cipher tools. Everything runs locally — nothing leaves your browser.