How to use the Caesar cipher tool
Type your message and choose a shift with the slider. With a shift of 3, A becomes D, B becomes E and so on, wrapping round so that X becomes A. The two rows under the slider show exactly which letter turns into which. Choose Decode to reverse the shift and read a message someone has sent you.
If you have a coded message but do not know the shift, press Try all 25 shifts. Every possible answer is listed, with the one that looks most like English at the top, so the real message almost always appears first.
Capital letters stay capital, and spaces, numbers and punctuation are left as they are, which keeps messages easy to read.
About the Caesar cipher
The Caesar cipher is one of the oldest known ciphers. The Roman historian Suetonius wrote that Julius Caesar protected his private letters by replacing each letter with the one three places further along the alphabet.
It is easy to use but also easy to break. With only 25 possible shifts, anyone can simply try them all, which is exactly what this tool does. That makes it a good first cipher for learning about codes, for escape rooms and for treasure hunts, but not for real secrets.
ROT13 is a Caesar cipher with a shift of 13. Because the alphabet has 26 letters, applying it twice gets you back where you started.
Questions and answers
How do I decode a Caesar cipher without the key?
Paste the message and press Try all 25 shifts. The answers are ranked by how much they look like English, so the right one is usually at the top.
What shift did Julius Caesar use?
According to the Roman historian Suetonius, a shift of three: A became D.
Is the Caesar cipher secure?
No. There are only 25 possible keys, so it can be broken in seconds. Use it for fun and puzzles, not for anything private.
What happens to numbers and punctuation?
They are left unchanged. Only the letters A to Z are shifted.