An early French digit cipher: deciphering a letter from the King of France to the Duke of Nevers (1592)
Date
2024
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Tartu University Library
Abstract
We deciphered a single letter written in 1592 by Henry IV, King of France, to Louis de Gonzague, Duke of Nevers, held in the Bibliothèque Nationale de France (BnF). The ciphertext mostly consists of contiguous digits, and demonstrates an early use of digit ciphers in 16th-century France. In this letter, Henri IV exposes some parts of his current military strategy against the Catholic League. After deciphering the letter, we were able to locate the original cipher table in another BnF manuscript, illustrating how codebreaking may assist historical research both to reconstruct the content of encrypted letters and to identify anonymous cipher tables.
Description
Keywords
homophonic ciphers, digit ciphers, continuous digit ciphers, French cipher, King Henry IV, the Duke of Nevers, the French Wars of Religion, BnF