ROT13 (“rotate by 13”) is a simple letter-substitution cipher that replaces every letter with the one 13 places later in the alphabet. Because the alphabet has 26 letters, applying ROT13 twice returns the original text — so a single tool both encodes and decodes. It is widely used to hide spoilers and puzzle answers, not for real security.
How it works
For each letter, ROT13 shifts it 13 positions forward, wrapping around the end of the alphabet. Since 13 is exactly half of 26, shifting forward 13 and shifting back 13 land on the same letter — that is why the operation is its own inverse. The tool preserves case (uppercase stays uppercase, lowercase stays lowercase) and leaves digits, spaces, and punctuation untouched, rotating only the letters A–Z and a–z.
Example
Encoding Hello, World!:
Uryyb, Jbeyq!Run that result through ROT13 again and you get Hello, World! back. Note the comma, space, and exclamation mark are unchanged.
ROT13 alphabet reference
| Plain | A B C D E F G H I J K L M |
|---|---|
| ROT13 | N O P Q R S T U V W X Y Z |
| Plain | N O P Q R S T U V W X Y Z |
|---|---|
| ROT13 | A B C D E F G H I J K L M |
The symmetry of the table shows why ROT13 is self-inverse: the second row of the first half matches the first row of the second half exactly.
Where ROT13 is actually used
Usenet and forums. ROT13 originated on Usenet in the 1980s as a convention for hiding spoilers, offensive jokes, and puzzle answers. Readers who have not solved a puzzle yet scroll past the ROT13 block; those who want to see it decode it manually or with a tool.
Capture-the-flag (CTF) challenges. ROT13 and its generalisations appear in beginner-level CTF puzzles as an introduction to classical ciphers. It is often the first cipher a participant is expected to recognise and reverse.
Obfuscating email addresses against scrapers. Before modern spam filtering, some websites encoded email addresses in ROT13 in HTML source to slow down simple scraping scripts. The technique is largely obsolete now.
Punchline delay in text. Some comedians and writers use ROT13 in the body of long-form text to place the punchline at the end of the encoded string, forcing the reader to decode it deliberately.
What ROT13 is not
ROT13 provides exactly zero cryptographic security. Any 13-year-old with pencil and paper can decode it in minutes by frequency analysis — or just try all 25 shifts. Never use it to protect anything that matters. For scrambling text that needs to stay hidden, use a real encryption tool.
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