How the Caesar cipher works
The Caesar cipher - named for Julius Caesar, who used it for his private correspondence - shifts every letter a fixed number of places down the alphabet. With a shift of 3, A becomes D, B becomes E, and HELLO becomes KHOOR. To decode, you shift back by the same amount. Letters wrap around the end of the alphabet (with shift 3, Z → C), and spaces, numbers and punctuation are left untouched.
What is ROT13?
ROT13 is just a Caesar cipher with a shift of 13 - exactly half the 26-letter alphabet. That makes it its own inverse: apply ROT13 twice and you’re back where you started, so the same button encodes and decodes. It’s the classic way to hide spoilers and punchlines in plain sight. Pick 13 · ROT13 in the shift selector above.
How do I decode a Caesar cipher without the key?
There are only 25 possible shifts, so a Caesar cipher is trivially broken by trying them all. Hit Show all 25 shifts and read down the list until one line turns into real words - that’s your message. (This is exactly why the Caesar cipher is a fun puzzle, not real security.)
The wheel that made it famous: Read the story: Decoder Ring History →
Want the whole toolkit? Open the full Codes & Ciphers console →