At the time of the German attack on Denmark and Norway 9 April 1940, Germany demanded that there should be no interruption of their telecommunications transmitted over Swedish lines. After a positive answer, the Germans gradually hired lines in Sweden for the connections Oslo - Copenhagen - Berlin, Oslo - Trondheim, Oslo - Narvik and Oslo - Stockholm. The German embassy in Stockholm already possessed a line for their traffic to Berlin. Later on they hired a line for the connection Stockholm - Helsingfors. The interception of the German telegraph lines was the fundamental condition for the future successful breaking of the German encrypted messages. The Swedes had, by these means, access to large quantities of genuine information. Wartime arrangements allowed foreign rented telecommunication lines passing through Swedish territory to be tapped, without breaking Swedish law.
We shall here for the first time in open literature show the principles involved in the breaking of the Geheimschreiber. The Germans considered this cipher machine to be extremely secure, but their confidence resulted in an imaginary security. German carelessness and Professor Arne Beurling's genius exposed the secret of the Geheimschreiber.