Dutch Cryptanalysis of Four American Diplomatic Codes in World War I

Abstract

"During the First World War, the Netherlands carefully maintained a neutral position. To guard this neutrality, the Dutch authorities monitored the activities of the belligerent surrounding countries. International telecommunications via telephone and telegraph were closely monitored and censored by censorbureaus.

In 2019, the Dutch intelligence and security service released a dossier about these censorbureaus to the Dutch National Archive. In that dossier, a previously unknown history of two groups of pioneering codebreakers based at the censorbureaus in Amsterdam and Rotterdam, was uncovered. In 2024, a first publication appeared about this dossier, with particular emphasis on how the local staff successfully broke German codes.

Additionally, the Dutch codebreakers successfully broke four American diplomatic codes between June and December 1918. This breakthrough enabled Dutch intelligence to monitor secret diplomatic traffic between American officials in the Netherlands and Washington during and after World War I.

This paper examines the systematic cryptanalysis of U.S. Department of State communications by Dutch codebreakers. Through analysis of original documents and surviving codebooks, it identifies the compromised diplomatic codes and places these findings in a broader historical perspective."