Caesar Cipher Tool

Encode or decode text with the classic Caesar shift cipher.

With Caesar Cipher Tool, you can do this check or generate this value in seconds, with no account or install needed. Caesar Cipher Tool makes this small but important security step practical to actually do. Everything runs in your browser — your file or text is never uploaded to a server.

Step-by-step

Type the text you want to encode or decode into the Text field.
Set Shift (1-25) to how many places each letter moves along the alphabet. Whoever decodes the message needs to know this number, since it's the key.
Under Mode, choose encode to scramble a message or decode to read one.
Click Run and copy the result. When decoding, use the same shift that was used to encode.

Example

With a shift of 3 in encode mode, ABC becomes DEF, because every letter moves three places forward. The alphabet wraps around at the end, so with the same setting XYZ becomes ABC. A teacher covering ciphers might hand students a sentence encoded with a shift of 5 and ask them to crack it without the key. Once they realize there are only 25 possible shifts, trying each one finds the message in minutes. It's a good way to show why the method offers no real protection.

A short history and the logic of the Caesar cipher

The Caesar cipher is one of the oldest encryption methods, shifting every letter a fixed number of places along the alphabet. According to ancient sources Julius Caesar used a shift of 3 in his correspondence. The key is the shift amount, so there are only 25 possible keys.

Why is it not secure?

All 25 keys can be tried by hand in minutes.
Letter frequency analysis finds the key: the most common letter in the language is also the most common in the ciphertext.
If word spaces and punctuation are kept, the pattern is even easier to crack.

Educational uses

Code-breaking exercises in a cryptography class
ROT13 (shift 13): encoding and decoding are the same operation because 13+13=26. It is used on forums to hide spoilers.
Secret-message games with children

For real privacy use modern, trusted encryption instead of this tool. Test a short word first to see how accented characters are handled.

Things to keep in mind

The classic Caesar cipher works on the 26-letter Latin alphabet. Test a short word first to see how accented or non-English letters are handled.
A shift of 13 is known as ROT13. Because the alphabet has 26 letters, applying it twice returns the original text.
Numbers, spaces and punctuation usually stay as they are, which reveals word lengths and sentence structure in the encoded text.
In a long encoded text, the most frequent letter often gives away the shift. That's a fair hint to give if you're building a puzzle.