ROT13 decoder and encoder

Paste on the left and read the result on the right. The same step encodes and decodes, so the swap button is switched off. There's nothing for it to do.

Shift moves the letters by any amount from 1 to 25, which turns this into a general Caesar cipher, and Auto-detect works the shift out from the text. Numbers decides whether digits sit still or rotate by five, a convention puzzle setters call ROT5. Below the tool you'll find all 25 shifts ranked, so you can read the answer off even when the shift is a mystery.

ROT13 replaces each letter with the one 13 places further on, wrapping from Z round to A. Thirteen is half of 26, so doing it twice lands you back where you started. One operation, no key, no separate decoder.

It isn't protection and was never sold as any. Recognise the method and you've broken it, and the cipher identifier recognises it in about a millisecond.

ROT13 grew up on Usenet in the early 1980s as the polite way to post a spoiler or a punchline. The text stayed visible but stopped being readable by accident, and every newsreader had a key that flipped it back. The problem it solved was social, not cryptographic.

It has also turned up where something stronger was wanted. Windows XP stored its record of the programs you'd run in a registry key obfuscated with ROT13, apparently to deter a casual snoop. It deterred nobody: the trick was spotted and documented, and the key has been readable ever since.

Atbash is the other fixed substitution here, mirroring the alphabet rather than rotating it, and it undoes itself the same way.

Not letters at all? Base64 and binary are what unreadable usually turns out to be, and the cipher identifier checks both alongside the shifts.