The Vigenère Cipher: A story of Cryptography and Intrigue
The Vigenère Cipher was one of the first polyalphabetic ciphers that were documented. It first appeared in the 1585 book Traicté des Chiffres (A Treatise on Secret Writing) by Blaise de Vigenère. Although Giovan Batista Belaso is considered to be the original creator of the cipher, he discussed the cipher in a booklet ‘La cifra del. Sig.’ (1553).
Vigenère took inspiration from the works of Alberti, Trimethius, and Porta. Leon Battista Alberti discussed a cipher that used a metal cipher disc to switch between cipher alphabets after several words, and switches were indicated by writing the letter of the corresponding alphabet in the ciphertext. Johannes Trimethius in his work Polygraphiae invented ‘tabula recta’ which is a critical component in Vigenère Cipher.
Giovanni Battista della Porta invented the Porta Cipher which made use of 13 alphabets in which the enciphering and deciphering methods are the same.
This cipher was used heavily by the Confederate States of America during the American Civil War, in which they used a brass cipher disc to implement the Vigenère Cipher.
For many years, this cipher was considered impregnable and was known as ‘le chiffre indéchiffrable’ which translates to ‘the unbreakable cipher’.
The cipher works by substituting each individual character and is therefore known as a polyalphabetic cipher. To unscramble the message, the intended receiver needs to know which row of the Vigenère square has been used to encipher each letter, so there must be an agreed-upon system of switching between rows, this is done by the use of a keyword.
It is essential that the length of the keyword be the same as that of the message; this is achieved by repeating the keyword multiple times until it reaches the length of the message. Vigenère also described an auto-key system using which the message itself is added to the keyword to make it the length of the message.
In order to encode a message using Vigenère Square, a letter from the message and a keyword are taken. Using the message letter as the column index and the keyword letter as the row index, the intersection is obtained.
Similarly, this process is repeated for every letter of the message.
To decrypt, select a letter from the key and the ciphertext from the row that corresponds to the key letter. Find the letter in the ciphertext, and the corresponding column will be the message letter.
Plaintext : CRYPTOGRAMMER
Keyword : VICTORY
Ciphertext : DLMUTRQYUAREUMathematical Description:
Ei = (Pi + Ki)mod 26
Di = (Ei — Ki)mod 26
Although this cipher was considered unbreakable, it is known to have been broken by Charles Babbage as early as 1854, although he did not publish his work.
In 1861, Friedrich W. Kasiski published the solution to the repeated key Vigenère ciphers based on the fact that identical pairings of the message and key symbols generate the same cipher symbols.
Try out Vigenère Cipher!
