Autokey Cipher

An extension of the running key cipher which appends the plain text message to the keyword.

This removes the weakness that a repeating keyword introduces, but adds in its own flaw.

Flaws

• Since the keyword contains the plain text, it will likely contain commonly used words. This can be used to partially decode the cipher text. By selecting the most promising results, more of the keyword can be deduced. This can result in cracking the cipher.