BOOK OF CODES AND CIPHERS

8- Vigenere Cipher

The VIGENÈRE CIPHER is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. It is a simple form ofpolyalphabetic substitution.

The Vigenère (French pronunciation: ​[viʒnɛːʁ]) cipher has been reinvented many times. The method was originally described by Giovan Battista Bellaso in his 1553 book La cifra del. Sig. Giovan Battista Bellaso; however, the scheme was later misattributed to Blaise de Vigenère in the 19th century, and is now widely known as the "Vigenère cipher".

Though the cipher is easy to understand and implement, for three centuries it resisted all attempts to break it; this earned it the description le chiffre indéchiffrable (French for 'the indecipherable cipher'). Many people have tried to implement encryption schemes that are essentially Vigenère ciphers. Friedrich Kasiski was the first to publish a general method of deciphering a Vigenère cipher in 1863.

Blaise de Vigenère developed a square to help encode messages. Reading along each row, you can see that it is a really a series of Caesar ciphers the first has a shift of 1, the second a shift of 2 and so.

The Vigenère cipher uses this table in conjunction with a key to encipher a message.

So, if we were to encode a message using the key COUNTON, we write it as many times as necessary above our message. To find the encryption, we take the letter from the intersection of the Key letter row, and the Plaintext letter column.

To decipher the message, the recipient needs to write out the key above the ciphertext and reverse the process.

The maths behind the Vigenère cipher can be written as follows:
To encrypt a message:

Ca = Ma + Kb (mod 26)

To decrypt a message:
Ma = Ca – Kb (mod 26)

(Where C = Code, M = Message, K = Key, and where a = the ath character of the message bounded by the message, and b is the bth character of the Key bounded by the length of the key.)