ROT13
Apply ROT13 — the Caesar cipher with shift 13. Self-inverse: applying ROT13 twice returns the original text. Common in Usenet spoilers, puzzles, and very light obfuscation.
Rotate every letter by 13
A becomes N, B becomes O, M becomes Z, N becomes A — and round and round. Apply ROT13 a second time on the output and you get the original input back.
Hello, World!
Uryyb, Jbeyq!
What you'll use this for
ROT13 has been the internet's go-to "polite spoiler hider" since the early days of Usenet. It's not security — it's a convention that says "I'm hiding something; opt in to read it."
Spoiler text
Hide spoilers on Usenet, forums, or anywhere readers can ROT13 it back if they want to peek.
Hiding punchlines
Joke setups in the open, punchlines in ROT13 — readers choose when to be surprised.
CTF puzzles
A staple of capture-the-flag challenges that test pattern recognition.
Education
Teach the simplest substitution cipher.
How to apply ROT13
Paste your text
Drop it into the left editor. Letters get rotated by 13; everything else passes through unchanged.
Toggle options if needed
Choose whether to preserve uppercase / lowercase, and whether to auto-run on input changes.
Click Run ROT13
Or leave auto-run on for live updates. Runs locally — no upload.
Copy or download
Copy to clipboard or save the result as rot13.txt. Run ROT13 again on the output to recover the original.
Frequently asked questions
Because 13 + 13 = 26 = the alphabet length, applying it twice cycles back to the original.
Absolutely not. It's a fixed-shift substitution; trivial to read.
Yes.
Pass through unchanged.
ROT13 is a Caesar cipher with a fixed shift of 13. Caesar Cipher lets you pick the shift.
About ROT13
ROT13 ("rotate by 13 places") is a special case of the Caesar cipher. Each letter of the English alphabet is replaced by the letter 13 positions later, wrapping around at the end. It came to wide prominence on the Usenet newsgroup net.jokes in the early 1980s, where it became the convention for hiding spoilers, punchlines, and offensive jokes so that readers could choose whether to decode them.
Why it's its own inverse
The English alphabet has 26 letters. A shift of 13 moves you to the opposite side of the alphabet, and a second shift of 13 moves you another 13 places — back to where you started, because 13 + 13 = 26 ≡ 0 (mod 26). That means the same function encrypts and decrypts: no need for a separate "decode" mode, just pipe the ciphertext back through ROT13.
Modern uses
- Spoilers and punchlines on forums, mailing lists, and Usenet, where the convention is well known.
- Obfuscating answers in puzzles, quizzes, and CTF challenges.
- Teaching simple substitution ciphers and modular arithmetic.
- Light text obfuscation — but never for anything that needs real privacy.
For an adjustable shift, use the Caesar Cipher. For actual encryption, use a modern algorithm like AES.