What it does
The Caesar cipher replaces each letter with the letter a fixed number of places further along the alphabet:
- Encrypt moves each letter forward by the shift, wrapping round from Z to A.
- Decrypt moves each letter back by the same shift, so the encrypted text reads as your text again.
Capital letters stay capital and small letters stay small.
How to use it
- Choose Encrypt or Decrypt and set the Shift, then type or paste into “Your text”. Select “Try a sample” to see an example.
- The result appears next to your text and updates as you type.
- Select Swap to move the result into “Your text” and switch the mode, for example to check that your message decrypts back.
- Select Copy to put the result on your clipboard.
The rules
- Only the 26 letters A to Z and a to z are shifted. Accented letters, other alphabets, digits, spaces, punctuation, emoji and line breaks stay exactly where they are.
- The shift is a whole number from 1 to 25. A shift of 13 is ROT13, which undoes itself.
- To read a message when you don’t know the shift, choose Decrypt and try each shift until the text makes sense.
Limits
Your text can be up to 1,000,000 characters long.
Privacy
Your text is processed on your device: the shifting runs in your browser, and nothing you type or paste is uploaded. Only the mode and shift you chose are kept in your browser for next time.