Autokey
Vigenère's self-extending key — the message becomes its own key.
Crib propagation and language-scored searches; ordinary repeating-key Kasiski analysis does not apply directly.
Why This Matters
Plaintext autokey extends a primer with the message itself. It avoids cycling a short keyword at a fixed period, but letters and passages can still repeat. This changes the attack surface rather than proving 350 years of field security. Joshua Holden’s The Mathematics of Secrets (2017), §5.3, distinguishes several historical self-keying constructions.
Cardano, Bellaso, and Vigenère discussed different self-keying and polyalphabetic ideas. The familiar repeating-key “Vigenère” cipher is associated with Bellaso’s 1553 work; Vigenère’s 1586 book included autokey constructions. The widget teaches the specified keyword-plus-plaintext model, rather than reproducing every feature of those historical systems.
Pick a short keyword K. Encrypt the first |K| plaintext letters with K under a Vigenère shift. Then take the plaintext itself, prepend K, and use that string as the running key for the rest of the message. Decryption recovers K-many letters first, then uses those recovered plaintext letters as the key for the next stretch — and so on. There is no fixed cycled keyword after the primer. The appended message still contains language structure and may repeat.
The extended key is related to the plaintext and is not independent random material. A guessed plaintext letter reveals a later key letter, allowing the guess to propagate at a distance equal to the primer length. Crib testing and language scoring can exploit this structure; there is no universal guarantee of recovery within a specified number of trials.
| Concept from Autokey | Modern Evolution |
|---|---|
| Self-extending key | CFB feeds ciphertext back through a keyed block cipher; that is distinct from appending bare plaintext as a keystream |
| Friedman's index of coincidence (κ) | Still the canonical periodicity test taught in cryptanalysis courses |
| ‘Don't reuse keys’ isn't enough | If your ‘key' has natural-language structure you've leaked it |
Ciphertext: QOXSXPWRYOSKP
Hint: the primer is a single Renaissance-style keyword, but the rest of the key comes from the plaintext itself.
| Era | Renaissance · 1586 |
| Trust | Broken |
| Origin | Blaise de Vigenère (1586), refining Belaso/Cardano |
| Year | 1586 |
| Key Type | Short keyword, then plaintext itself extends the key |
| Broken By | Crib propagation and statistical analysis |
| Modern Lesson | Self-keyed schemes still leak: the key is no longer random once it carries plaintext statistics |