An Autokey cipher is identical to the Vigenère cipher with the exception that instead of creating a keyword by repeating one word over and over, the keyword is constructed by appending the keyword to the beginning of the actual plaintext message.
For example, if your plain text message was:
This is a secret message
And your keyword was "zebra", then your actual keyword would be:
zebrathisisasecretmessage
Enciphering and deciphering the message is performed using the exact same method as the Vigeńere Cipher.
