The Caesar cipher is one of the oldest and simplest ways of scrambling a message. Each letter is replaced by the one a fixed number of places further along the alphabet. With a shift of 3, A becomes D, B becomes E, and “ATTACK” becomes “DWWDFN”. The Roman historian Suetonius records that Julius Caesar used a shift of three for private letters, and that Augustus used a shift of one.
Type your message, choose Encode or Decode, and set the shift.
How the tool works
Letters move along by the shift and wrap around at the end of the alphabet, so with a shift of 3, X becomes A. Capitals stay capitals and lowercase stays lowercase. Everything else, including spaces, digits, punctuation and accented letters, is left alone, which is why a Caesar-encoded sentence still shows its word lengths and punctuation. That is part of what makes it weak.
To decode, choose Decode with the same shift that was used to encode. Decoding with a shift of 3 is the same as encoding with a shift of 23, since 3 plus 23 is a full trip around the 26 letters.
Cracking a message
If you have a Caesar-encoded message and no key, tick Try all 25 shifts. The tool shows every possible decoding in a table, and one of them will read as proper text. This takes seconds, and it is the fatal weakness of the cipher: with only 25 possible keys, anyone can try them all. A cleverer approach, known since the 9th century when the Arab scholar Al-Kindi described frequency analysis, uses the fact that letters are not equally common in a language. In English, “e” is the most frequent letter, so the most common letter in a Caesar-encoded message is probably “e” shifted by the key.
ROT13
A shift of 13 has a special property: because the alphabet has 26 letters, encoding twice returns the original. It is called ROT13. It was used on Usenet to hide spoilers and punchlines, so that readers had to make a deliberate choice to read them. It was never intended as security.
Do not use it for anything private
A Caesar cipher is a game, a teaching example and an occasional element in puzzle hunts and escape rooms. It is not secure for any purpose, and a determined child can break it. For real protection of text, the protected text tool uses strong encryption, which is a different thing altogether.
For other classic codes, try Morse code, the binary translator or the Pig Latin translator. If a message looks like a jumble of the right letters, the anagram checker helps with a different sort of puzzle.