Caesar Cipher Encoder & Decoder

The Caesar cipher is a substitution cipher that replaces every letter in a message with another letter a fixed number of positions further along the alphabet. Shift by three and A becomes D, B becomes E, and Z wraps around to C. It is named after Julius Caesar, who according to the Roman historian Suetonius used a shift of three to protect his military correspondence. This tool encodes plain text into ciphertext, decodes ciphertext back to plain text, and can try every possible shift at once when you do not know the key.

A Caesar cipher has only 25 usable keys, one for each shift from 1 to 25, which is what makes it easy to learn and easy to break. Because the number of keys is so small, an attacker can simply try them all, an approach called brute force. The tool builds that full list for you: paste an intercepted message, and it shows all 25 candidate decodings so you can read down the column and spot the one that turns into real words. This is why the Caesar cipher is a teaching example today rather than real security.

Two well-known special cases are included. ROT13 is a Caesar cipher with a shift of exactly 13, and because 13 is half of 26, applying it twice returns the original text, so the same button both scrambles and unscrambles. It is used on forums and in Usenet posts to hide spoilers and punchlines without truly encrypting them. ROT47 extends the idea to 94 printable ASCII characters, so it also rotates digits and punctuation. Everything here runs entirely in your browser: your text is never uploaded, logged, or stored, the tool works offline once loaded, and it is free with no account required.