Variant Beaufort

History/Description

Polyalphabetic substitution ciphers are based on a system described by the accomplished Italian cryptographer Leon Battista Alberti (1404-1472) in his treatise De Cifris published around 1466 and called the Alberti Cipher disk. This used two concentric disks to encipher consecutive letters with different alphabets making it much more difficult to decipher without the key. A similar method was later described by the Italian Giovanni Baptista della Porta (1535-1615) in his book De Furtivis Literarum Notis in 1563. The Porta cipher consists of 13 cipher alphabets each of which is selected by either of two key letters. The encipherment is a reciprocal process with the plaintext being found in the table header (if A-M) or the row corresponding to the key letter (N-Z) and the cipher letter is in the same column but the opposite pair. So if the key is C and the plaintext letter is H the cipher letter is V.

A few years after the invention of the Porta cipher, the Frenchman Blaise de Vigenère (1523-1596) invented a similar cipher called the Autokey which he described in his book Traicté des Chiffres in 1585. However he's more famously remembered for the cipher named after him which uses a similar table to the Porta cipher but with 26 alphabets. To encipher text using the Vigenère cipher the key letter is found in the left column of the table and determines the row, and the plaintext letter is found in the top row and determines the column. The intersection of these reveals the cipher letter. So if the key is C and the plaintext letter is H the cipher letter is J.

Two hundred years after the Vigenère cipher was invented Irish born Rear Admiral Sir Francis Beaufort (1774-1857) published two variations of the Vigenère; the Beaufort and Variant Beaufort ciphers in 1857. The Beaufort cipher uses reversed alphabets for both the key and ciphertext in the table whereas the Variant Beaufort uses just a reversed key. Using the Beaufort cipher if the key is C and the plaintext letter is H the cipher letter is V whereas with the Variant Beaufort cipher the ciphertext letter is F.

These ciphers remained largely unbroken for over 300 years until the infantry officer and cryptologist Major Friedrich Wilhelm Kasiski (1805-1881), who was born in Prussia (now Poland), published his book Die Geheimschriften und die Dechiffrierkunst ("Secret Writing and the Art of Deciphering") in 1863. This described a general solution for polyalphabetic substitution ciphers which is now known as the Kasiski method or test.