Codes and Ciphers :: Autokey Cipher
To encrypt a plaintext message using the Vigenère Cipher, one locates the row with the first letter to be encrypted, and the column with the first letter of the keyword. The ciphertext letter is located at the intersection of the row and column. This continues for the entire length of the message.
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:
Enciphering and deciphering the message is performed using the exact same method as the Vigenère Cipher.
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