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Vigenere Cipher Explained

Games : Codes And Ciphers

Historically many ciphers have existed, for instance the specific Caesar shift cipher, the standard monosubstitution cipher and ciphers containing monosubstitution mixed with void characters and the odd code word.

One of the biggest developments was the Vigenere cipher which was thought to be unbreakable, until it was cracked finally.
This code is still often set as a challenge for experienced code breakers and requires a lot of skill to break.

It is also a common task for computing students in degree level IT to have to write a program to decrypt or encrypt a piece of text using this cipher.

The Vigenere cipher has a keyword that defines how to look up data from a table, called the Vigenere table, that is a Ceasar Shift. Each row has an extra shift from 0 through to a shift of 25.

First of all the keyword is written, and repeated above the message in the plaintext as many times as necessary. Then to find the cipher, the look up is simply performed from the intersectoin of the row of the keyword letter and the column of the plaintext letter.

Therefore this is much more secure than a monosubstitution cipher because various letters each represent the one initial letter, depending which letter of the keyword is above the letter in the plain text.

However the downside is that because there is a keyword there are patterns that emerge in the ciphertext, and by finding how frequently those patterns occur one can guess the length of the keyword and then try to deduce what the keyword is.

Note that the longer the keyword, the more secure it is, and if the keyword is meaningless this is also a great help. If the keyword is as long as the text itself, then that would give maximum security.


By: Fred

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