ROT13 decoder with all 25 shifts on screen

No — it is not encryption, and calling it a cipher is generous. There is no key: knowing the method is knowing the secret, so anyone who recognises rotated text can undo it, and the panel on this page breaks every possible shift at once in a few milliseconds. Even without a tool, ROT13 falls to a glance at letter frequency, since the shape and spacing of the words survive intact. Use it to cover a spoiler that a reader chooses to reveal, never to protect a password, a licence key or anything you would mind a stranger reading.

Paste it in and read the output — decoding is the same operation as encoding, so there is nothing to switch. That is the whole trick of thirteen: the alphabet has 26 letters, thirteen is exactly half, and moving a letter half a turn twice brings it back to the start. A becomes N and N becomes A. It is the only rotation with that property, which is why ROT13 became the one everybody uses and why every tool that offers it needs only one button.

Because 13 is half of 26, and only half a turn makes the operation its own undo. Any other shift needs its opposite to reverse it — text hidden with ROT7 has to be read with ROT19 — which means two buttons, two commands to remember and one more thing to get wrong. Thirteen removes all of that. The same arithmetic gives ROT47 its shift: there are 94 printable ASCII characters, 47 is half of them, so it undoes itself the same way, and ROT5 does the job for the ten digits.

Nothing at all on the default setting — only the 26 unaccented English letters move, and everything else is passed through exactly as typed. Capitals stay capital, so Hello becomes Uryyb rather than uryyb. This is deliberate: leaving the punctuation in place is what keeps a rotated sentence recognisable as a sentence. If your text has digits that also need hiding, the second mode shifts them by five, and ROT47 goes further and scrambles brackets, quotes and every other printable ASCII character too, which makes the output look like line noise rather than like a coded message.

From Usenet in the early 1980s, where the newsgroup net.jokes adopted it as a courtesy. Posts with punchlines that would spoil a film, give away a riddle or offend somebody were rotated so that reading them took a deliberate act, and newsreader programs bound the operation to a single keystroke, making it the only cipher in common use with its own keyboard shortcut. The convention spread to spoiler warnings across the early internet and outlasted the newsgroups themselves. It is still doing that job: geocaching.com stores the hint for every cache in ROT13 so that a searcher who wants to keep trying is not spoiled by seeing it.

Yes — ROT13 is a Caesar cipher with the shift fixed at thirteen. Suetonius describes Julius Caesar using a shift of three in his private letters, writing D where he meant A, and records that Augustus preferred a shift of one and wrote AA when he reached the end of the alphabet. What Caesar had that we do not is an audience that mostly could not read at all, let alone attack a substitution. The method was broken for good in the ninth century, when the Arab polymath al-Kindi described frequency analysis and showed that counting letters undoes any simple substitution.

Then it is not a Caesar cipher, and the list of 25 has just ruled out an entire family in one look. Try the ROT47 setting first, especially if the text is full of punctuation and symbols. If the text is a long run of letters and digits ending in one or two equals signs it is probably Base64 rather than a rotation; if it is pairs of hex digits it is bytes, which the text to hex page will read back. Text where each letter maps to a different letter every time it appears is a Vigenère cipher and needs the keyword, and nothing on this page will get it out.