ROT13 Encoder & Decoder

ROT13 (rotate by 13) is a simple letter substitution cipher: A→N, B→O, ..., M→Z, N→A, ..., Z→M. Numbers and non-letters pass through unchanged. Because 13 is exactly half of 26, applying ROT13 twice returns the original text.

ROT13 was widely used on Usenet newsgroups in the 1980s–90s to hide spoilers, punchlines, and content some readers might find offensive. It's not encryption in any security sense — it's trivially reversible — but remains useful for casual spoiler-hiding, puzzle creation, and as an introduction to substitution ciphers.

Our decoder is identical to the encoder — just run it again to reverse. Works on both uppercase and lowercase; punctuation and spaces pass through untouched.

The number 13 is the entire trick. The alphabet has 26 letters, and 13 is exactly half, so shifting forward by 13 and shifting back by 13 land in the same place — ROT13 is the only letter rotation that is its own inverse. A ROT5 or ROT20 would need a separate decoder; ROT13 needs one button. That symmetry is why it, and not some other shift, became the internet's standard way to hide text in plain sight.

ROT13 is the modern descendant of the Caesar cipher — Julius Caesar is reported to have protected his correspondence with a three-place shift two thousand years ago. The difference is intent: Caesar wanted secrecy, while ROT13 deliberately offers none. It is a curtain, not a lock — the point is that the reader must choose to decode it, which is exactly right for spoilers and puzzle answers.

It is still in daily use. Geocaching hints are traditionally published in ROT13 so players can choose when to peek, and puzzle communities use it the same way. It also produces accidental poetry: a few English words ROT13 into other English words — 'irk' becomes 'vex', and 'abjurer' becomes 'nowhere'.