Caesar Cipher / ROT13

What Is a Caesar Cipher?

A Caesar cipher replaces each letter with the letter a fixed number of places further along the alphabet. With a shift of 3, A becomes D, B becomes E and Z wraps round to C. It is named after Julius Caesar, who is said to have used it in his letters. ROT13 is the same idea with a shift of 13 - and because the alphabet has 26 letters, applying it twice gives back the original text.

What It Is Used For

• Hiding spoilers and puzzle answers in forums - ROT13 is the classic way.
• Teaching the basics of cryptography in class.
• Escape room clues and treasure hunts.

How to Use It

1. Type or paste your text.
2. Choose the cipher: a Caesar shift you set, ROT13, ROT18 (ROT13 plus digits) or ROT47 (all printable ASCII characters).
3. For a Caesar shift, choose the shift and whether to encode or decode.
4. Don't know the shift? Click Try All 25 Shifts and look for the row that reads as normal words.

Example

With a shift of 3, Meet me at the old library at noon. becomes Phhw ph dw wkh rog oleudub dw qrrq. Capital letters stay capital, and spaces and punctuation are not changed. Decoding with the same shift returns the original sentence.

ROT13, ROT18 and ROT47

ROT13 changes letters only, so numbers stay readable. ROT18 also rotates digits by five. ROT47 rotates every printable ASCII character, including digits and symbols, so Hello, World! becomes w6==@[ (@C=5P.

Limitations

These ciphers are not encryption. There are only 25 possible shifts, so anyone can read a message in seconds - never use them for passwords or private information. Only the 26 English letters are shifted; accented letters and other scripts are left as they are. To protect real data, use a modern tool such as the Secure Password Generator; for another classic code, try the Morse Code Translator.