A related question: how did the former German Intelligence officers react when it became known that the British had been running every singe one of their agents and feeding them dis-information in operation XX (Double-Cross)?

edit all the German agents in Britain, I mean.

The announcement of Enigma in 73 came in a book by Gustave Betrand: Enigma: ou, La plus grande énigme de la guerre 1939-1945. To be honest, it seemed to go mostly unremarked at the time. It really took F.W. Winterbotham's book "The Ultra Secret" in 1974 to really rock the boat. This became big news at the time, at least in the UK. Winterbotham wrote from the viewpoint of an ageing participant from the administrative side (albeit with the key responsibility of protecting the distribution) without any access to notes or knowledge of the technology, for more see Here [PDF warning] is a nice little article assessing the impact from the NSA. Anyway, Winterbotham's book opened the flood gates and others followed with more details, particularly Cave Brown's Bodyguard of Lies. The latter book suggested that Dönitz had suspicions about the security of the Enigma machine but even during post-war interrogations [PDF warning] but it doesn't appear that anyone had more than the vaguest of supicions. When Winterbothams book first appeared, it seems that the Germans wrote it off. When Cave Brown's book appeared, this was no longer possible.

I can't find a reference at the moment to what the surviving Germans felt when the Cave Brown book appeared but note that there was a war game of operation sealion (invasion of the UK) by the British and West German militaries at the RMA Sandhurst in 1974 and this totally ignored Bletchley Park's capabilities at the time.

Edit: I attributed something to Canaris that should be rightly to Dönitz.

We have had to remove a large number of comments in this thread which simply state "Why are all the comments gone!?" Please don't post that, as it will only compound the issue!

Here in /r/AskHistorians, we have certain standards that we require all top-level responses to meet. Comments which don't meet them get removed. In this thread, there have been a large number of incorrect, speculative, or otherwise disallowed comments, and as such, they were removed by the mod-team. I hope that satisfies your curiosity!

Also, was this an open secret, something suspected but with little evidence, or a complete surprise?

This question got me thinking, and then sent me back to my usual references on wartime British intelligence, and it strikes me: I don't think I've ever seen any substantive post-war reaction or analysis to British intelligence's success from the German side. And after a few text searches, I can't find any now.

Out of pure curiosity, though, I'd be particularly interested to know what Reinhard Gehlen (a Nazi-era military intelligence officer who went on to become the founding father of the West German intelligence service, BND) thought of these successes, but I can't find anything on that front right now.

As other commenters have noted, both the GC&CS Ultra programme which led to the cracking of German military communications and the Double-Cross system were closely-guarded secrets even after the war. Ultra didn't become public knowledge until the 1970s — and the revelation of GCHQ's existence (and GC&CS' wartime activities) caused a political scandal and the sensational 1978 prosecution under the Official Secrets Act (the ABC Trial) of two journalists and a former soldier.

As for the Double-Cross System, it only came to light in 1972 when former MI5 officer and Twenty Committee chairman J.C. Masterman published a book on the subject: The Double-Cross System in the War of 1939–45. Masterman's decision to publish the book all but destroyed his relationships with many erstwhile comrades in the intelligence world, who regarded it as a profound betrayal of their duty to keep their secrets.