Autokey Cipher is an improvement to #Vigenère Cipher that not only use keyword but also plaintext as the part of the key generation which is as long as the plaintext. By knowing the keyword, we can recover the rest of the message as the plaintext is also part of the key. This makes it rather resilent to Kasiski Method#, however due to its inclusion of plaintext as part of the key, it is not entirely safe from Frequency Analysis#.