Browsing by Author "Lasry, George"

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    An early French digit cipher: deciphering a letter from the King of France to the Duke of Nevers (1592)
    (Tartu University Library, 2024) Desenclos, Camille; Lasry, George; Waldispühl, Michelle; Megyesi, Beáta
    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.
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    Deciphering Historical Syllabic Ciphers
    (Tartu University Library, 2024) Lasry, George; Waldispühl, Michelle; Megyesi, Beáta
    Historical ciphers with syllabic elements are significantly more challenging for cryptanalysis than regular homophonic ciphers. We present here a novel computerized technique which recovers significant parts of the keys, allowing for the remaining parts to be manually completed. We solved several previously undeciphered French, Spanish, and Italian syllabic ciphers, and we also evaluated the performance of this method against a series of additional historical syllabic ciphers.
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    What Encryption Errors Can Reveal: Cross-Cipher Errors in Mary Queen of Scots’ Letters
    (Tartu University Library, 2024) Biermann, Norbert; Tomokiyo, Satoshi; Lasry, George; Waldispühl, Michelle; Megyesi, Beáta
    In the recently deciphered letters from Mary Queen of Scots, a large number of systematic encryption errors were found and attributed to confusion as a result of concurrently using at least one other cipher key to communicate with a different recipient. In this paper, we further analyze such cross-cipher errors in those letters and identify additional cipher keys involved. This analysis also reveals valuable insights on the secret communications of Mary, Queen of Scots. We employ several techniques including statistical analysis, which may be applied to the analysis of encryption errors in other collections of historical enciphered documents.