December 4, 2009

An autokey cipher uses the plain-text of the message being sent to form part of the key. There are many kinds of autokey ciphers. The one we will examine is classic, invented by Blaise de Vigenère in 1586, over four hundred years ago.

De Vigenère’s autokey begins with a keyword, then appends to the keyword the plain-text of the message being sent. Each letter of the plain-text is then enciphered by indexing through the alphabet according to the corresponding letter of the key:

plain-text  A C O L L E C T I O N O F E T U D E S
key         P R A X I S A C O L L E C T I O N O F
cipher-text P T O I T W C V W Z Y S H X B I Q S X

Decryption is the inverse operation.

Your task is to write functions that encipher and decipher a message as shown above. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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