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ROT13

Rotate by thirteen — run it twice and you are back where you started

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One operation, both ways

Thirteen is half of twenty-six, so rotating twice returns the original. There is no decode button because there is nothing for it to do — paste encoded text in and the plain text comes out.

Input

Rotated

Copied!

Not a cipher. A politeness protocol that happens to look like one

ROT13 has never been about secrecy and nobody who used it thought it was. It appeared on Usenet in the early 1980s as a courtesy: a way to post a punchline, a spoiler or an answer so that it did not arrive in your eye before you had chosen to read it. The reader who wanted it typed a key; the reader who did not, did not. It is a curtain, not a lock, and every newsreader of the era had a single keystroke to pull it back.

The mechanism is one property of the number 26. Rotating a letter thirteen places and then thirteen more moves it twenty-six places, which is all the way round the alphabet and back to where it started. So ROT13 is its own inverse: the encoder and the decoder are the same operation, there is no second function, and no direction has to travel alongside the message. That is why this page has one button rather than an encode/decode toggle — a decode toggle would be a control that does nothing.

It is still in daily use for exactly the job it was invented for. Puzzle communities post hints in it, mailing lists hide answers in it, some CTF and adventure-game hint systems still serve it, and it survives in the classic tr a-zA-Z n-za-mN-ZA-M one-liner that half of Unix's users know by heart. It has also earned an ironic second life as the standard example in any conversation about security theatre.

So: do not use it to protect anything. It has no key, its algorithm is universally known, and reversing it is a single automatic step — which is the whole point, since a reader has to be able to undo it trivially for the convention to work at all. If you need something a person cannot casually read, that is not what a rotation cipher is for either; it is a job for real encryption.

Run it twice and the text comes back

In
Spoilers ahead: the butler did it.
Out
Fcbvyref nurnq: gur ohgyre qvq vg.

ROT13 questions

Why is there no decode button?

Because there is nothing for it to do. Thirteen is half of twenty-six, so rotating by thirteen twice takes every letter all the way around the alphabet and back to itself. The encoder is the decoder. Paste encoded text into the same box and the plain text comes out — a separate decode mode would be a second button running identical code.

Is ROT13 secure?

Not remotely, and it was never meant to be. There is no key, the algorithm is public, and undoing it is one automatic step — which is the requirement, not the flaw, because the reader is supposed to be able to reveal a spoiler effortlessly. It is a curtain over a punchline. For anything that actually needs protecting, use encryption.

What about numbers — is there a ROT5?

Yes. ROT5 does the same trick to the ten digits, since five is half of ten, and ROT18 is the two combined so that letters and numbers both rotate and both self-invert. ROT47 extends the idea across most of the printable ASCII range including punctuation. All three are the same one idea applied to a different alphabet size.