The Vigenere Cipher, created by Blaise de Vigenre* of France in the 16th century, is an example of a poly-alphabetic shifted cipher. Unlike the traditional mono-alphabetic shifted ciphers, which shift every character in a message by the same amount (such as the Caesars Cipher), the characters in a message of a Vigenere Cipher is to be shifted by different amounts. This may be achieved by using a word or a phrase as the encryption key.

For example, if the word key is to be used for encryption: Message: ENCRYPTIONEXAMPLE Key: KEYKEYKEYKEYKEYKE Encryption: ORABCNDMMXIVKQNVI

E is the 5th letter of the alphabet and K is the 11th letter. The resulting encryption will be E shifted by 11, or the 15th letter of the alphabet O. The rest of the message will follow the same encryption algorithm. In another example, suppose that the plaintext to be encrypted is: ATTACKATDAWN

The person sending the message chooses a keyword and repeats it until it matches the length of the plaintext, for example, the keyword "LEMON": LEMONLEMONLE

Each row starts with a key letter. The remainder of the row holds the letters A to Z (in shifted order). Although there are 26 key rows shown, you will only use as many keys (different alphabets) as there are unique letters in the key string, here just 5 keys, {L, E, M, O, N}. For successive letters of the message, we are going to take successive letters of the key string, and encipher each message letter using its corresponding key row. Choose the next letter of the key, go along that row to find the column heading that matches the message character; the letter at the intersection of [key-row, msg-col] is the enciphered letter.

For example, the first letter of the plaintext, A, is paired with L, the first letter of the key. So use row L and column A of the Vigenre square, namely L. Similarly, for the second letter of the plaintext, the second letter of the key is used; the letter at row E and column T is X. The rest of the plaintext is enciphered in a similar fashion:

Plaintext:	ATTACKATDAWN
Key:	LEMONLEMONLE
Ciphertext:	LXFOPVEFRNHR

Decryption is performed by going to the row in the table corresponding to the key, finding the position of the ciphertext letter in this row, and then using the column's label as the plaintext. For example, in row L (from LEMON), the ciphertext L appears in column A, which is the first plaintext letter. Next we go to row E (from LEMON), locate the ciphertext X which is found in column T, thus T is the second plaintext letter.

ASSIGNMENT. Create a .txt, .doc, .pdf or similar file. Name the file as YourLastName Vigenere. In the top left corner of your document type your full name and the words Vigenere Cipher. Write your answers to the two challenges below, and submit your completed document to this assignment forum.

Given the key and ciphertext, solve the plaintext message:

Plaintext:	____________________________________________
Key:	CYBERCYBERCYBERC
Ciphertext:	RYMSRNRPRVVUPVBU

Given the plaintext and ciphertext, solve for the key:

Plaintext:	FIREWALL
Key:	______________________________________________
Ciphertext:	XMTYNIEJ