Only Gentlemen Read Each Other’s Mail: Over 50 Years of Sittler Codebooks in the Dutch Diplomatic Service

Abstract

The Dutch diplomatic service’s prolonged reliance on commercially available Sittler codebooks, despite repeated indications the books had been compromised, presents an interesting case study spanning the late nineteenth century to the early 1930s. With the use of the Sittler code as its focal point, this article explores how cryptographic practices were shaped as much by social norms, administrative capacities and financial constraints as by awareness of strategic security risks. Drawing primarily on archival material from the Dutch Ministry of Foreign Affairs, it demonstrates that continued use of the Sittler code reflected neither ignorance nor technological backwardness. Instead, it exposes a class-based, “gentlemanly”, conception of secrecy in which codes fulfilled a primary function of shielding information from subordinate staff and telegraph personnel, rather than from senior officials of foreign governments. Only under the external pressures of the First World War did concerns of foreign cryptanalytic threats lead to incremental security measures.