Bellaso and Vigenère: the story of a mistaken attribution

For more than three centuries handbooks called the polyalphabetic system with a literal key, long considered unbreakable, the “Vigenère cipher”. The name, however, is the result of a misunderstanding. The method had been published in Venice in 1553 by a gentleman from Brescia, Giovan Battista Bellaso, in a booklet entitled La cifra del Sig. Giovan Battista Bellaso, while the French diplomat Blaise de Vigenère described a different form of it only in 1586.

Bellaso's countersign

Relatively little is known about Bellaso: he came from a noble family of Brescia, studied law and worked as a secretary in the service of senior prelates, including, it appears, Cardinal Rodolfo Pio di Carpi. Like many secretaries of the time he handled confidential correspondence and knew the limits of the nomenclators then in use. His innovation was practical: entrusting the change of alphabet to a word or phrase that was easy to remember, which he called the contrassegno, or countersign.

The principle, in descriptive terms, is this: each letter of the message is paired with a letter of the countersign, repeated as often as needed, and each letter of the key indicates which alphabet to use. Sender and recipient need only share the word, which can be changed at will without replacing tables or booklets. Bellaso later published two more booklets, in 1555 and 1564, with variants and refinements of his system.

Vigenère's Traicté

Blaise de Vigenère, born in 1523, was a secretary and diplomat in the service of the French court and stayed in Rome, where he came into contact with the circle of papal cipher clerks. In 1586 he published in Paris the Traicté des chiffres, a large treatise that gathers with great erudition the history of secret writing, mingling it with reflections on Kabbalah, alchemy and the mysteries of numbers.

Vigenère's most original contribution is not the repeating-key cipher but a more refined system called the autokey: a short initial key starts the encryption, and then the message itself supplies the subsequent letters of the key. It is a sturdier method, but it was soon forgotten. In the nineteenth century Vigenère's name became attached to the simpler version, Bellaso's.

The chiffre indéchiffrable and its fall

The repeating-key cipher acquired the reputation of le chiffre indéchiffrable, the indecipherable cipher. In reality its weak point was precisely the repetition of the key: if the word has five letters, every fifth letter of the text is enciphered with the same alphabet. Once the length is found, the message breaks down into a set of simple substitutions, open to frequency analysis.

The English mathematician Charles Babbage solved the problem around the middle of the nineteenth century but did not publish his method. In 1863 the Prussian officer Friedrich Wilhelm Kasiski explained in a book how to find the length of the key by observing repeated sequences in the ciphertext. From then on the cipher lost its aura, although it remained in use for less important communications.

The rediscovery of Bellaso is owed to twentieth-century historians of cryptography, who reread his booklets and restored to the man from Brescia the credit for the invention. His story reminds us that the history of ideas is also made of wrong names, and that a practical innovation, a keyword easy to remember, can be worth more than many complications.