Although some people had suspicions, particularly Karl Dönitz the head of the Kriegsmarine, the full story of the breaking of Enigma (Ultra) was not finally revealed to the world until 1974 when The Ultra Secret was published by F.W. Winterbotham. However, other works hinting at the fact that Enigma had been broken, such as Battle for Secrets by Władysław Kozaczuk and The Codebreakers by David Kahn, had been published before that in 1967. During the war, the Germans dismissed evidence that Enigma had been broken.
Indeed, Kapitän zur See Heinz Bonatz, the former head of the Kriegsmarine's B-Dienst (Beobachtungsdienst), part of German Naval Intelligence, asserted in his history of German naval intelligence, Die deutsche Marine Funkaufklärung 1914–1945, published in 1970, that Enigma had never been broken.
Enigma decrypts (Ultra) were shared with a very small and select audience and the British went to great lengths to disguise their sources, both from their own people and from the Germans. Intelligence was generally only acted on when its source could plausibly be ascribed to some other source, such as a fake reconnaissance flights that “discovered” the target independently.
As a result, the Germans had suspicions that Enigma might have been broken but never had a smoking gun that proved that it had been. The Germans were not aided in their efforts by the highly fragmented nature of their own intelligence operations and by the, ever present, inter-service lack of cooperation.
Enigma in use
The German’s were aware that information was leaking to the Allies, but they tended to ascribe the leaks to other sources: agents (for instance Rudolf Roessler and the Lucy spy ring), new technology, or random bad luck, etc. Of all the services, the German Navy (Kriegsmarine) appears to have been the most security conscious and they made several improvements to their encryption machinery (four rotor Enigma) and procedures which caused the code-breakers at Bletchley Park major headaches.
Both Admiral Kurt Fricke and Konteradmiral Ludwig Stummel of B-Dienst, conducted investigations into the possibility that Enigma had been compromised, specifically into the sinking of the merchant raider Atlantis (Schiff 16) in November 1941 by the heavy cruiser Devonshire (which was indeed the result of an Enigma decrypt). However, these investigations concluded that Enigma had not been compromised.
When the Germans investigated various incidents where Enigma could have been compromised, they concluded that Enigma was not at fault. For instance, when convoys avoided Wolfpacks, the Germans assumed that the cause lay with Huff-Duff (high-frequency direction finding) and radar rather than Enigma.
In 2001, a book, Action This Day: Bletchley Park from the Breaking of the Enigma Code to the birth of the Modern Computer, by Michael Smith and Ralph Erskine, reported that an American-Swiss spy in the US Navy Department had informed the Germans that Enigma had been broken for several months. However, the German investigation concluded that although specific, short-lived, keys may have been captured, Enigma itself was not compromised.
It wasn’t the case that the Germans thought that Enigma couldn’t be broken, they were well aware that, theoretically at least, it could be broken. What they failed to assess correctly was the extraordinary effort that the British were prepared to put into breaking the code and the technical innovations that they developed in order to do so (the Bombes, etc). It’s not that the Germans didn’t understand that there was a risk, it’s simply that they gave it a very low priority in their list of things to worry about.
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