Deciphering a Letter from the French Wars of Religion

Abstract

A collection held at the Biblioth`eque Nationale de France contains the deciphered version of messages related to negotiations of the Catholic League, Henry III’s enemy, with Spain and the Catholic Church in Rome. At least one of those messages was deciphered by Vi`ete. Two other letters in the collection contain enciphered passages without the corresponding plaintext. Using computerized techniques, the author deciphered one of them, from Claude de Bauffremont, baron de Sennecey, ambassador of the League in Rome. In this article, the author describes the process of recovering the key, and of deciphering most parts of the letter, which includes a report of Sennecey’s activities in Rome. The cipher used to encode this letter turnout out to be a homophonic cipher with a relatively high number of homophones, making its codebreaking somehow challenging.