Because it is half of twenty-six, which is what makes encoding twice return the original.

No. It offers no security and was never intended to — it hides text from casual reading, nothing more.

Text Transform

ROT13 decoder

Your text

What to do

Caesar shift

Result 0 → 0 characters · 0 lines

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Local · its own inverse, by construction

ROT13 shifts every letter thirteen places. Because thirteen is exactly half of twenty-six, applying it twice returns the original — so encoding and decoding are the same operation.

How to use the rot13 decoder

1 Paste your text into the first box.

2 Choose what you want done to it.

3 Copy the result, or save it as a text file.

That self-inverse property is the entire reason ROT13 became a convention rather than a curiosity. Usenet groups used it for spoilers, punchlines and offensive jokes precisely because a reader had to take a deliberate action to see the text, and any client could implement both directions with one function. It offers no security whatsoever and never claimed to — it is a politeness mechanism, the text equivalent of turning a page upside down. Numbers are untouched, which is why the ROT47 variant exists for anyone who wants digits and punctuation scrambled too.

Questions

How do I decode ROT13? −

Apply ROT13 again. It is its own inverse, so the same operation does both.

Why thirteen? +

Is ROT13 encryption? +

Why was it used on Usenet? +

Does it change numbers? +

Sources

Britannica — Caesar cipher

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