Caesar cipher decoder

How the Caesar cipher works

Pick a number from 1 to 25. That's the key. Move every letter of the message that many places down the alphabet, wrapping from Z back round to A. With a shift of 3, A becomes D, X becomes A, and TREASURE MAP becomes WUHDVXUH PDS.

To decode, shift back by the same amount. A shift of 3 forward is undone by 3 back, which is the same as 23 forward. That's why the tool above lets you encode or decode with one shift box.

Cracking it without the key

There are only 25 possible keys. The table under the tool tries every one and ranks them by how much the result reads like English. We score each candidate on four-letter sequences: TION and THER are common in English, QXZJ never appears. The right shift almost always comes out on top, even for messages a few words long.

If none of the 25 reads, it isn't a Caesar cipher. The two usual suspects are a general substitution, where the letters are scrambled instead of shifted, and Vigenère, which is a Caesar shift that changes with every letter.

Spotting a Caesar cipher

• Word lengths, spacing and punctuation look normal, but the words don't.
• Short words give it away. A one-letter word is A or I shifted, so a lone D suggests a shift of 3 and a lone L a shift of 3 from I.
• The letter frequencies have the same shape as English, just slid sideways. Frequency analysis shows it clearly on longer messages.

Where you'll meet it

Suetonius says Julius Caesar wrote to his generals with a shift of three. Two thousand years later it's still the first cipher most people learn and the first layer of a lot of puzzle hunts. It's often the outer wrapper on a harder cipher, so if your decode gives you more gibberish, run it through the cipher identifier next.