Tools · Classical ciphers
Caesar cipher online
Encode and decode the Caesar cipher and ROT13, try every shift at once and spot the one that reads.
Runs in your browser. Your text goes nowhere.
How to use it
Paste a text and choose a shift. The result appears as you type. The “⇅” button swaps the fields and flips the mode, which is a handy way to check that decrypting gives back the original.
The alphabet is detected automatically: if there are more Cyrillic letters, the text is treated as Russian (33 letters, including ё), otherwise as Latin (26 letters). Digits, spaces and punctuation stay as they are, and so does letter case.
When the key is unknown
The brute-force panel tries every shift at once and ranks the results by how much they look like real language. It uses χ², the mismatch between the text's letter frequencies and those of ordinary English or Russian. For a phrase of five or six words the right answer is almost always on top. For very short strings, read the first three or four rows yourself.
ROT13
ROT13 is a Caesar cipher with a shift of 13 over the Latin alphabet. Since 13 + 13 = 26, the same operation both encrypts and decrypts. Press the ROT13 button and the shift and alphabet are set for you.
Questions
What shift did Caesar himself use?
Three: A → D, B → E. Suetonius says so, and adds that Caesar's successor Augustus shifted by one.
How do you decrypt a Caesar cipher without the key?
By brute force: there are only 25 keys for the Latin alphabet (32 for Russian). The table below does it for you. More in the lesson on the Caesar cipher.
Is it safe to type text here?
Yes. Everything runs in your browser; nothing is sent to the server.