Cryptography in Theory and Practice: the German-French Context (1300-1800)

deadline for submissions: 
April 30, 2023
full name / name of organization: 
University of Heidelberg

International Conference

Heidelberg, 11-12 April 2024


In the pre-modern era – as today – the successful transmission of messages played a central role both in public and private life. Functioning administration, effective implementation of political decisions, coordination of military operations, trade, or even the maintenance of long-distance interpersonal contacts were hardly manageable without written communication. Naturally, for the senders and recipients it was important that the information transmitted would only be accessible to those for whom it was explicitly intended. But this was by no means a given. For as long as there have been postal systems, there has also been postal espionage. The most common means of protecting the contents of a letter from unauthorized reading was to encrypt everything, or at least the most sensitive passages.

Authors of cryptological manuals and treatises tried to satisfy the need for increasingly sophisticated methods of securing information. Literature containing instructions on how to encrypt or decode texts has existed since the 16th century at the latest. In this respect, research is mainly interested in the development of the polyalphabetic substitution, with the thesis that the most important inventions fell into the period before 1600. Katherine Ellison paints a different picture in her study of seventeenth-century English cryptography manuals (A Cultural History of Early Modern English Cryptography Manuals, 2017). Not only were the respective methods quite innovative and original; they also reflected the authors’ efforts to appeal to a broad readership with varying background knowledge. Ellison also notes that the manuals were aimed at a society that was struggling with “crises of expression” (p. 1) in a time of political, social, religious, and scientific unrest caused by the Civil War. Historians also sporadically examine the relationship between cryptographic theory and practice – however, much research is still required in this regard.

In recent years, cryptographic practice has also increasingly become the focus of research. A conference organized by Anne-Simone Rous in Gotha (Germany) in 2013 as well as the anthology co-edited with Martin Mulsow (Rous/Mulsow: Geheime Post, 2015) were important impetuses for this trend. This is probably also where the foundation stone was laid for a more far-reaching research network, in which mathematicians, computer scientists, and linguists interested in historical cryptography joined forces under the name HistoCrypt ( The papers presented in the course of the HistoCrypt conferences mainly concern the resolution of not yet decrypted cryptograms, the analysis of cryptographic techniques or the presentation of interdisciplinary projects such as the Decode Database ( In particular, the work on the architecture of early modern keys carried out under the direction of Beáta Megyesi has considerably advanced our knowledge of the subject.

While a monograph on cryptological practice is now available at least for Hungary (Benedek Láng, Real Life Cryptology, 2018), larger and/or cross-century studies are still lacking for the rest of Europe. Thus, large parts of the history of early modern cryptography remain in the dark. The aim of the planned conference is to shed light on these at least in part – for a German-French context – and thus to initiate new research. Special attentions will be given to three desiderata of historical cryptography research.

The first area concerns the cryptological instructional literature. While the English manuals are comparatively well researched, there is still a lack of in-depth historical contextualizations of works published in Germany and France. Not only the authors, their professions and networks are of interest, but also their motivations behind the publication of cryptological literature, as well as the groups of people and contexts they were intended for. Furthermore, it will be asked how the respective authors tried to grasp cryptology taxonomically and terminologically as a subject matter, how they systematized the books, how they reflected and optimized existing cryptological knowledge, and how they conveyed this knowledge medially and rhetorically (e.g., graphics, tables). The tension between cryptographic theory and practice will also be explored. The focus will not only be on the extent to which literature affected cryptographic practice: As practice and theory always condition and shape each other, it will also be asked how cryptographic practice and contemporary socio-cultural conditions affected literature.

Second: Due to the evident transmission bias of encrypted diplomatic records, research has so far naturally concentrated on diplomatic correspondence. Far less attention was paid to the administrations – first and foremost the higher-level line ministries – whose task it was to prepare the keys. At times, there were even departments specifically entrusted with encryption – in France, for example, the French Foreign Ministry’s „bureau du chiffre“ in the eighteenth-century –, an activity, of which we know almost nothing about. Who, for example, prepared the keys for the diplomats sent by France to the Old Empire or to Europe, according to which they had to encode their correspondence? What about the imperial estates, where the bureaucratization of the ministries’ work was not infrequently less pronounced or where there were far fewer staff than in France? Moreover, little is known about state-organized postal espionage, an activity that made text encryption necessary. Although there have been attempts to research these spy centers known as „cabinets noirs”, for example through Stewart Oakley’s pioneering work on Celle (The Interception of Posts in Celle, 1694-1700, 1968) or Karl de Leeuw’s studies on postal espionage in the Netherlands (The Black Chamber in the Dutch Republic during the war of the Spanish Succession and ist aftermath, 1707-1715, 1999), comparable investigations for other countries have yet to be conducted. Even with regards to France, where postal espionage is believed to have been widespread and systematic from Louis XIV’s reign, there is no recent work, apart from Eugène Vaillé’s book published in 1950 (Le Cabinet noir, 1950). Finally, with the exception of Láng’s work, it should also be noted that research has yet to shed light on encryption practices in personal and business letters.

Third: We also have only incomplete information about central aspects of the encryption practice: According to what rules and when did the letter writers decide to encrypt part of their correspondence and leave the rest in plain text? What instructions did they receive from their ministries and cabinets? Was ciphering practice also related to the status of the sender (general, ambassador, envoy, agent, resident, consul)? Were there other reasons besides the importance or secrecy level of a piece of information, such as institutional, political, or personal, for encoding passages of text? Could it be that encryption practices were, in some instances, a matter of pure habit and personal routine? Did the encryption frequency also depend on the availability of a secretary? What role played the postal routes chosen for dispatch? The quantitative studies carried out so far on individual correspondences or correspondence series (according to chronological or geographical aspects) have not allowed the elaboration of model-like development schemes. Analyses based on qualitative aspects, on the other hand, have shown that encryption always depended heavily on the political context at the place of dispatch.

Our conference hopes to address all these questions. Due to the intensity of Franco-German relations since the late Middle Ages as well as the research desiderata mentioned above, a geographical focus will be placed on examples from this context. The organizers therefore welcome proposals for papers dealing with cryptographic theory and practice in Germany or France, or cryptography in the context of Franco-German relations.

Considering these research perspectives, preference will be given to papers that address the following questions:

  • History of cryptography literature (authors, contexts of origin, intended audience, structure, as well as systematization, reception, development, and communication of techniques).
  • Studies of the influence of cryptography literature on practice and vice versa.
  • Management of cryptography and postal espionage in ministries.
  • Activities of the Black Chambers (postal espionage practices).
  • Development of encryption techniques.
  •  Case studies: quantitative and qualitative analysis of coded passages in correspondences.

Abstracts of the proposed papers (400 words with a brief curriculum vitae) may be submitted in German, French or English to the following address: by April 30, 2023. The scientific advisory board of the meeting will decide on the selection of contributions during the month of May 2023.


French, German, English



Camille Desenclos (Université de Picardie Jules Verne / CHSS, EA-4289,

Sven Externbrink (Universität Heidelberg,

Eveline Szarka (Universität Heidelberg,

Jörg Ulbert (Université Bretagne Sud, Lorient / TEMOS, UMR-9016,


Scientific Advisory Board

Dejanirah Couto (École pratique des hautes études / SAPRAT, EA-4116)

Camille Desenclos (Université de Picardie Jules Verne / CHSSC, EA-4289)

Sven Externbrink (Universität Heidelberg)

Benedek Láng (Eötvös Loránd University)

Beáta Megyesi (Uppsala Universitet)

Eveline Szarka (Universität Heidelberg)

Jörg Ulbert (Université Bretagne Sud / TEMOS, UMR-9016)