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David Kahn wrote that "Delastelle invented a fractionating system of considerable importance in cryptology." This was the bifid cipher, which combined fractionation with transposition as opposed to substitution, as was done by Pliny 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.
Delastelle's work on the four-square cipher was published in a book in 1902, and was 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 Chase in 1859.
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 until 1860. After leaving school, he worked in the local port, as a bonded warehouseman, for forty years, and pursued his interest in amateur cryptography as a hobby.
Following his 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.
- Kahn, David, The Codebreakers, page 243