Félix Delastelle

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Félix Marie Delastelle (1840–1902) was a Frenchman most famous for his invention of several systems of polygraphic substitution ciphers including the bifid, trifid, and the four-square ciphers.

Delastelle's work on the four-square cipher was published in a book in 1902, though some of Delastelle's ideas were anticipated by an American mathematician and astronomer named Pliny Chase in 1859. "Delastelle invented a fractionating system of considerable importance in cryptology." (Kahn The Codebreakers, page 243). The names of the ciphers associated with Delastelle are principally the Bifid cipher and the Trifid cipher. His Four-square cipher is a variant on the earlier Playfair cipher.

Delastelle may have been unaware of Playfair, but he had read of the fractionating cipher described by Pliny Earle Chase in 1859. The first presentation of the bifid appeared in the French Revue du Génie civil in 1895 under the name of cryptographie nouvelle.

There are few biographical details. Félix-Marie's father, a master mariner, was lost at sea in 1843. Félix attended the College of Saint-Malo and then worked in the local port as a bonded warehouseman for forty years. After retirement, in 1900, he rented a single room in a holiday hotel where he wrote a 150 page book Traité Élémentaire de Cryptographie which he completed in May 1901. On hearing news of his brother's sudden death, he collapsed and died in April 1902. His book appeared three months later, published by Gauthier-Villars of Paris.

Delastelle is unusual for being an amateur cryptographer at a time when significant contributions to the subject were made by professional soldiers, diplomats and academics.