All operations

ROT13

Shift every letter thirteen places. Encoding and decoding are the same operation.

What it does

ROT13 replaces each letter with the one thirteen places further along the alphabet. A becomes N, B becomes O, and so on past Z and back round to A.

Thirteen is exactly half of twenty-six, which gives ROT13 its one memorable property: applying it twice returns the original text. There is no separate decode step, and no key to lose. The same operation runs in both directions.

What it is for

Hiding a punchline, a spoiler or a puzzle answer from someone reading casually. That is the whole intended use, and it has been the convention on Usenet and forums since the early 1980s.

It provides no security whatsoever. Anyone who recognizes it can reverse it in their head, and any tool will strip it instantly. If you need something unreadable to an attacker rather than merely unreadable at a glance, you want encryption, not a letter shift.

The Caesar cipher is the general form, where you choose the shift distance. ROT13 is simply the Caesar cipher with a shift of thirteen. Atbash shares the self-inverse property by mirroring the alphabet instead of rotating it.