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The Vigenère autokey cipher fixes the biggest weakness of the standard Vigenère cipher — a short, repeating keyword — by extending the key stream with the plaintext itself once the keyword runs out, so the key never repeats.
This tool encrypts or decrypts text using a Vigenère autokey cipher with a keyword you choose.
How it works
Your keyword supplies the first few shift values, the same way it would in a standard Vigenère cipher. Once the keyword is used up, the key stream continues with the message's own letters (the plaintext) instead of repeating the keyword — so a long message never has a short, repeating pattern an attacker could exploit. Decryption works in the same self-extending way: as each letter is decrypted, it immediately becomes available as key material for a later position.
- Enter mode.
- Enter text.
- Enter keyword.
- Click Calculate to see your results.
Examples
Keyword "QUEENLY"
"ATTACKATDAWN" encrypted with the keyword "QUEENLY" produces "QNXEPVYTWTWP" — notice the key stream for this message is QUEENLY followed by ATTACKA (the plaintext itself), not QUEENLY repeated.
Who should use it
- Puzzle and escape-room style word games with a stronger twist on Vigenère.
- Teaching why key repetition is a weakness, and how autokey schemes address it.
Industry applications
- Cryptography education
- Games and puzzles
Advantages
- Removes the repeating-key pattern that makes standard Vigenère vulnerable to Kasiski examination.
- A clear illustration of how a cipher's own output (or input) can extend its key material.
Limitations
- Still a classical cipher with no real security against modern cryptanalysis.
- A known first few plaintext letters can help an attacker start reconstructing the key stream.
Common mistakes to avoid
- Using a different keyword for decryption than the one used to encrypt.
- Confusing this with the standard Vigenère cipher — the key streams (and therefore the ciphertext) are different even with the same keyword and message.
Best practices
- Use the Vigenère autokey cipher for puzzles or to see how removing key repetition changes a classical cipher's resistance to pattern-based attacks.
Tips
- Compare this tool's output side by side with the standard Vigenère Cipher Tool using the same keyword and message — the difference shows exactly how the key stream diverges once the keyword runs out.