How to use the Caesar cipher
- Type a message and pick a shift. With shift 3, A becomes D, B becomes E and Z wraps round to C.
- Choose Encode to scramble or Decode to unscramble with the same shift. Capitals, spaces and punctuation are kept.
- Got a coded message but no key? Press Crack it. The tool tries all 25 shifts and picks the one that looks most like English, or open "All 25 shifts" and read them yourself.
About the Caesar cipher
The Caesar cipher is one of the oldest known ciphers. The Roman historian Suetonius wrote that Julius Caesar shifted letters by three to protect military messages. Each letter is replaced by the one a fixed number of places further along the alphabet. ROT13, a shift of 13, is its modern cousin: because the alphabet has 26 letters, applying ROT13 twice gives back the original, which made it popular for hiding spoilers and puzzle answers online.
It is wonderful for teaching, escape rooms, treasure hunts, geocaching and puzzles, but it is not secure. There are only 25 possible keys, and frequency analysis cracks it instantly: E is the most common letter in English, so the most common letter in the coded text probably stands for E. The "Crack it" button does exactly that, using a chi-squared comparison with English letter frequencies. For real secrets, use proper encryption, such as our File Encryption tool with AES-256.
How do I decode a Caesar cipher without the key?
Press "Crack it", or read through all 25 shifts; only one will make sense.
What is ROT13?
A Caesar cipher with a shift of 13. Encoding and decoding are the same operation.
Can it handle numbers?
Tick "Also shift digits" to rotate 0–9 by the same amount. Traditionally only letters are shifted.