Explain how you would break the Vigere Ciphere

Vignere Cipher

Our follow on topic is Vignere Cipher which is an interesting algorithm developed by French Cryptographer Vignere.  I will post more about the Vignere Cipher tomorrow and ways to hack the Vigenere cipher.

(You will run into different spelling of Vignere)

Vignere Cipher is a primitive Cipher and is not practical for the computing today but it uses an interesting substitution.  There is something called Vignere table:

The Vignere cipher requires a key of fixed length. For our example we are going to use the word CRYPTO. Please do not worry about upper case and lower case. 

The clear text we are going to encrypt is:

IT IS A GREAT DAY AT MARYMOUNT

What we do next is to paste the key below the clear text with all spaces removed. 

ITISAGREATDAYATMARYMOUNT <--------------- Clear Text

CRYPTOCRYPTOCRYPTOCRYPT <-------------- Key

To encrypt the letter "I" in the message, you would look at row "I" and column "C" - the "C" in CRYPTO since they lined up. The result is "K". Next, we move to the next character, which is a "T" for the row and an column "R". I will let you complete the rest.  

ITISAGREATDAYATMARYMOUNT <--------------- Clear Text

CRYPTOCRYPTOCRYPTOCRYPT <-------------- Key

KK                                                     <-------------- CIPHERED TEXT

Assume you know the length of the key, how would you break this encryption? Think about this question and come up with ideas and we'll use next week discussion to post your findings