What is an Autokey Cipher?

An Autokey Cipher is a substitution cipher that uses the plaintext itself as a key, along with a keyword placed at the beginning of the plaintext. Then, a Vigenère table is used to encipher the keyed plaintext.

Example

Plaintext: Whoever has made a voyage up the Hudson must remember the Kaatskill mountains.

Key: WINKLE

Encipher step:

Keyed Text: WINKLEWHOEVERHASMADEAVOYAGEUPTHEHUDSONMUSTREMEMBERTHEKAATSKILLMOU
Plain Text: whoeverhasmadeavoyageupthehudsonmustrememberthekaatskillmountains
Ciphertext: (coming soon) 

Vulnerabilities of Autokey Cipher

Since the key is in English, using short English words along the length of the cipher text could reveal likely English results. This can be used to guess the length of the keyword and ultimately reveal it.

How to Crack an Autokey Cipher

Use a common short word and try it out as the key text. Look for likely English results in the resulting plaintext. Use this to guess at the length of the keyword. Shift the likely results back to find the keyword at the beginning of the shifted plaintext.