Autokey Cipher
Qnxepk tm dcgn!
Encode text with a primer keyword
The autokey cipher strengthens Vigenère: instead of repeating a short keyword, it primes the keystream with your keyword and then continues it with the plaintext you are enciphering. Because the key never repeats, it resists the repeating-key (Kasiski) analysis that breaks plain Vigenère.
When to use this tool
Reach for the autokey cipher when a puzzle or CTF challenge uses a keyword that does not simply repeat — the autokey (or autoclave) cipher primes the keystream with a short keyword and then continues it with the plaintext itself, so the key never repeats and the repeating-key analysis that breaks plain Vigenère no longer applies. It uses the same tabula recta shifts as Vigenère (A=0 … Z=25), so if a Vigenère decode stalls after the first few letters, an autokey primer is the natural next thing to try. Pick a primer keyword, then encode or decode on a single page.
Privacy and limitations
Everything runs locally in your browser; nothing is sent anywhere. This is a classical polyalphabetic cipher, not real encryption — it implements the plaintext (Vigenère) autokey only, transforms A–Z and a–z, leaves digits and symbols untouched, and offers no key-autokey (ciphertext) variant or auto-solving features. You still need the exact primer keyword to recover a message.