Caesar Cipher Encoder / Decoder

How the Caesar cipher works

Every letter is moved the same number of places along the alphabet, wrapping from Z back to A. That number — the shift — is the whole key. With a shift of 3, A becomes D, B becomes E, and X, Y and Z wrap round to A, B and C. Decoding moves each letter back by the same amount.

Suetonius records Julius Caesar writing to his generals with a shift of three, and Augustus using a shift of one. ROT13 is the same cipher with the shift fixed at 13.

A worked example

With a shift of 3, Attack at dawn becomes Dwwdfn dw gdzq. Case, spaces and punctuation are left alone, which is also the first thing that gives the cipher away: the word lengths of the original survive.

How it is broken

There are only 25 possible shifts, so the reliable attack is not cleverness but patience: try every one. Turn on try all 25 shifts above and the tool lists the message under each; exactly one row reads as English. That took a codebreaker a few minutes with a pencil, and it is why the cipher protects nothing.

The older attack is frequency analysis, first written down by al-Kindi in the ninth century. In English text E is the most common letter, so the most common letter in a long Caesar ciphertext is very likely E shifted — and that one guess gives away the shift for the whole message.

Everything on this page runs in your browser — nothing you type is sent anywhere. The cipher encoder / decoder runs all five ciphers on one page, and the cipher buttons above switch this one too.