One of the main problems with simple substitution ciphers is that they are so vulnerable to frequency analysis. Given a sufficiently large ciphertext, it can easily be broken by mapping the frequency of its letters to the know frequencies of, say, English text.

 Therefore, to make ciphers more secure, cryptographers have long been interested in developing enciphering techniques that are immune to frequency analysis. One of the most common approaches is to suppress the normal frequency data by using more than one alphabet to encrypt the message.

Vigenere Cipher is a POLYALPHABETIC substitution method involves the use of two or more cipher alphabets. Instead of there being a one-to-one relationship between each letter and its substitute, there is a one-to-many relationship between each letter and its substitutes.

The Vigenere Cipher, proposed by Blaise de Vigenere from the court of Henry III of France in the sixteenth century, is a POLYALPHABETIC substitution based on the following tableau:

Note that each row of the table corresponds to a Caesar Cipher. The first row is a shift of 0; the second is a shift of 1; and the last is a shift of 25.

The first column (in our table it’s red) always used to look for plaintext letter, the first raw (in our table it’s red also) always used to look for key letter, the letter drawn in between (with blue color) always used to look for the cipher.

We begin by writing the keyword, repeated as many times as necessary, above the plaintext message.

In encryption, to derive the ciphertext using the tableau, for each letter in the plaintext, one finds the intersection of the row given by the corresponding key letter and the column given by the plaintext letter itself to pick out the ciphertext letter.

In decryption, this time we use the key letter to pick a column of the table and then traces down the column to the row containing the ciphertext letter. The index of that row is the plaintext letter.

Vigenere like substitution ciphers were unbreakable for 300 years. In 1863, a Prussian major named Kasiski proposed a method for breaking a Vigenere cipher that consisted of finding the length of the keyword and then dividing the message into that many simple substitution cryptograms. Frequency analysis could then be used to solve the resulting simple substitutions.

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