What is a Caesar cipher?

A Caesar cipher is one of the oldest known encryption methods, named after Julius Caesar. It shifts every letter forward in the alphabet by a fixed number of places, wrapping Z back around to A. A shift of 3 turns A into D, B into E, and so on.

How do I decode a Caesar cipher?

To decode, shift each letter backward by the same key used to encode it. This tool shows a live round-trip decode so you can confirm the shift is reversible. A shift of 13 is its own inverse, which is the well-known ROT13.

Is a Caesar cipher secure?

No. With only 25 possible shifts it can be broken instantly by brute force or by frequency analysis. It is a teaching tool and a fun puzzle device, not a way to protect real secrets. Use modern encryption like AES for anything sensitive.

Does the cipher change my spaces and punctuation?

No. Only the 26 alphabet letters are shifted. Spaces, digits, punctuation, and symbols are left exactly where they are, which keeps the message readable as a puzzle while still hiding the words.

What is ROT13 and how is it related?

ROT13 is simply a Caesar cipher with a shift of 13. Because the alphabet has 26 letters, applying ROT13 twice returns the original text, so the same operation encodes and decodes. Set the shift to 13 in this tool to use it.

Caesar Cipher Text Generator

Caesar cipher text generator

A Caesar cipher is the classic substitution cipher that shifts each letter of the alphabet by a fixed amount. Type any message, choose a shift key from 1 to 25 (or roll a random one), and this tool produces the encrypted ciphertext in your browser instantly.

How it works

Each letter is replaced by the letter a fixed number of positions later in the alphabet. With a shift of k, the letter at position i (0–25) becomes position (i + k) mod 26. The modulo wraps the end of the alphabet back to the start, so with a shift of 3, x becomes a. Uppercase and lowercase are handled separately to preserve case, and any character that is not a letter is left untouched.

Decoding reverses the shift: position (i - k) mod 26. Because the operation is a simple rotation, applying a shift of k and then 26 - k returns the original text — that is the round-trip check shown beneath the output.

Tips and example

With a shift of 7, the word Gera becomes Nlyh. To decode it, shift back by 7. A shift of 13 is special: it is its own inverse, known as ROT13, so encoding and decoding use the exact same step.

Because there are only 25 usable shifts, a Caesar cipher is trivial to break and should never be used for real security. It is perfect, though, for escape-room puzzles, treasure hunts, geocaching clues, and teaching how substitution ciphers work.