About this Caesar cipher tool
The Caesar cipher is one of the oldest ways to hide a message. Every letter is replaced by the letter a fixed number of places further along the alphabet, wrapping from Z back to A, so with a shift of 3, A becomes D and HELLO becomes KHOOR. Julius Caesar is said to have used a shift of 3. This tool encrypts and decrypts with any shift, keeps upper and lower case, and leaves numbers and punctuation alone. Since there are only 25 useful shifts, it also tries all of them and compares letter frequencies with English, which is how a Caesar cipher is cracked.
How to use the Caesar cipher
- Enter your message. Type or paste the text, or press an example, which also sets the mode and shift.
- Choose the mode and shift. Pick Encrypt or Decrypt, then enter a Shift from 1 to 25 or press a common shift such as ROT13.
- Press Run cipher. The result appears with the cipher wheel, the step-by-step explanation, and the alphabet mapping.
- Crack an unknown message. If you do not know the shift, read the All 26 shifts table; the most English-looking line is marked with a star.
Frequently asked questions
How does a Caesar cipher work?
Each letter moves forward a fixed number of places in the alphabet and wraps around after Z. With a shift of 3, A becomes D, X becomes A, and Y becomes B. Decrypting moves the same number of places backward.
What is ROT13?
ROT13 is a Caesar cipher with a shift of 13. The alphabet has 26 letters, so applying it twice returns the original text, which is why the same step both hides and reveals a message.
How do I crack a Caesar cipher without the key?
Try every shift. There are only 25, and the All 26 shifts table lists them. The tool also scores each one against normal English letter frequencies and stars the best match, which works well for a sentence or more.
Are numbers and punctuation changed?
No. Only the letters A to Z move. Digits, spaces, punctuation, and accented letters stay exactly as they are, and upper and lower case are kept.
Is the Caesar cipher secure?
Not at all. With only 25 possible keys, anyone can try them all in seconds. Use it for puzzles, games, and teaching, and use modern encryption for anything private.