Caesar Cipher Encoder & Decoder

How the Caesar cipher works

The Caesar cipher is a substitution cipher: every letter of the message is replaced by the letter a fixed number of positions further along the alphabet. That number is the shift (or key). With a shift of 3, A becomes D, B becomes E, and so on; at the end of the alphabet you wrap around, so X becomes A, Y becomes B and Z becomes C.

To decrypt, shift back by the same amount. Spaces, digits and punctuation are left untouched, which is why the word shapes of the original survive in the ciphertext. With shift 3, HELLO becomes KHOOR; shifting KHOOR back by 3 returns HELLO.

In arithmetic terms, number the letters A=0 … Z=25; encryption is (letter + shift) mod 26 and decryption is (letter − shift) mod 26. That "mod 26" is the wrap-around.

A worked example

Notice that shift 25 is the same as shifting back by 1, and that the double letters (TT) stay double in every row. Patterns like that are exactly what makes the cipher breakable.

ROT13: the special case

ROT13 is a Caesar cipher with a shift of 13. Because the English alphabet has 26 letters, applying ROT13 twice brings you back to the original text, so the same operation both encrypts and decrypts. It has been used since the early days of online forums to hide spoilers, punchlines and puzzle answers: anyone can decode it instantly, but you have to choose to. Press the ROT13 button in the tool to set it.

History: Julius Caesar and Suetonius

The cipher takes its name from Julius Caesar, who, according to the Roman biographer Suetonius in The Twelve Caesars (written around AD 121), protected his private and military correspondence by replacing each letter with the one three places further along the alphabet. Suetonius adds that Caesar's nephew Augustus used a simpler version with a shift of one. For its time it was reasonably effective: most people could not read at all, let alone recognize a transformed Latin text.

Simple shift and substitution ciphers stayed in use for centuries, and in the 9th century the Arab scholar al-Kindi described frequency analysis, the systematic method for breaking them. By the Renaissance, serious secrets had moved to polyalphabetic ciphers like Vigenère's, but the Caesar shift lived on as a teaching example, a child's secret code and, as ROT13, a piece of internet culture.

Why it is weak

• Only 25 keys. There are 25 possible shifts (shift 0 changes nothing). Trying all of them by hand takes a minute; the brute-force panel above does it instantly. That is the whole reason the cipher cannot protect anything.
• Letter frequencies survive. In English, E, T, A and O are the most common letters. In a Caesar ciphertext the most common symbol is almost always E shifted by the key, so a single count reveals the shift without trying any.
• Word shapes survive. Spaces and repeated letters are preserved; a three-letter word with the pattern "XYZ" that appears everywhere is probably "THE".

None of this makes it useless. It is a perfect first cipher for teaching: it introduces keys, modular arithmetic, brute force and frequency analysis in one afternoon. Just never use it for anything you actually want to keep private.

Related ciphers and codes

The Vigenère cipher uses a keyword so the shift changes from letter to letter, defeating simple frequency counts. The Atbash cipher reverses the alphabet (A↔Z, B↔Y). And a code such as Morse or binary is a different thing altogether: it represents letters in another form without hiding them. A cipher conceals; a code transcribes. You can of course combine them, encrypting with Caesar and then sending the result in Morse, which is a classic scout and escape-room puzzle.