ROT13 Cipher - Encode & Decode Text

ROT13: the self-inverting cipher

ROT13 ("rotate by 13 places") is a monoalphabetic substitution cipher that replaces every letter with the one 13 positions later in the 26-letter Latin alphabet, wrapping past Z back to A. Formally, for a letter at zero-based indexi, the output index is (i + 13) mod 26. Because 13 is exactly half of 26, the transformation is its own inverse: applying it twice yields(i + 26) mod 26 = i, the original letter. That property is what makes ROT13 an involution, and why one button both encodes and decodes.

The mapping in full: A↔N, B↔O, C↔P, D↔Q, E↔R, F↔S, G↔T, H↔U, I↔V, J↔W, K↔X, L↔Y, M↔Z. Case is preserved and everything else - digits, spaces, punctuation, accented letters, emoji, CJK - passes through untouched. The classic demonstration isWhy did the chicken cross the road? → Jul qvq gur puvpxra pebff gur ebnq?. A well-known curiosity: PNG and ROT13 both encode to themselves' counterparts in amusing ways, and abjurer ROT13s tonowhere - one of only a handful of English word pairs that map onto each other.

Where ROT13 shows up:

• Usenet: the original use, hiding movie spoilers, punchlines, and offensive jokes so readers had to opt in; newsreaders like rn and trn shipped a one-key "R" decode
• Puzzle hunts and geocaching: geocaching.com still ROT13s every listed hint by default
• Config obfuscation: historically abused to "hide" strings in software - Windows XP shipped a registry key with ROT13'd values, which is a well-cited cautionary tale, not a pattern to copy
• Teaching: the standard first example when introducing substitution ciphers and frequency analysis

Why ROT13 is not encryption

ROT13 has a key space of exactly one. There is no secret, so there is nothing to guess - anyone who recognizes the format can reverse it instantly, and letter-frequency analysis gives it away even to someone who does not. It is closer to an upside-down answer key in a puzzle book than to a cipher. The running joke among cryptographers is that applying it twice gives you "ROT26", i.e. no encryption at all, and that triple-ROT13 is marketed as being as strong as single. Never use it for passwords, tokens, personal data, or anything an adversary should not read - use AES-256-GCM or a vetted library for that.

ROT13 sits inside the broader Caesar cipher family, where the shift can be any value from 1 to 25; Julius Caesar reportedly used a shift of 3. Only the shift of 13 is self-inverse, which is precisely why it became the internet's default. Related variants extend the idea: ROT5 rotates digits, and ROT47rotates 94 printable ASCII characters, so punctuation is scrambled too. Everything here is computed in your browser with a simple character map - nothing is transmitted or logged.