Why Didn't the Germans Know Enigma Was Broken?

The Enigma code was invented by Arthur Scherbius after the end of World War I, and the Enigma machine was invented by Hugo Koch five years later, about two decades before the start of World War II. It's main purpose was to protect the communications of business. The Polish had figured out how to translate the initial Enigma messages, but to up security during the war, the Germans changed the key list every day, which was distributed beforehand. This created an added challenge to breaking the code because it was never quite the same every day. Because of the sheer amount of possibilities, the Germans believed it to be unbreakable. However, it was broken by the British and the information gained was effectively used against the Germans. Yet, with the increasing success of the Allies, the Germans never found out that their code had been broken. When German Admiral Donitz, who commanded the German navy, was interviewed multiple decades after the war, he was unaware that the Enigma code had ever been broken.

There are multiple explanations as to why the Germans were naive towards the compromise of Enigma. The first explanation considers the probability of success when approaching the code with brute force, which can be deemed impossible through a few logical arguments. One of the biggest arguments considers the amount of time it would take to perform such a feat with the technology that was available at the time. If one were to use 100'000 Enigma machines to test a new key each second for every second in a year, it would take about twice the age of the universe to break it, clearly more time than they were allotted. Because of the impossibility of breaking Enigma through brute force, the Germans held a lot of confidence that it was completely unbreakable. And to an extent, Enigma should have been unbreakable, but unfortunately for the Germans, many encryption operators were sloppy with their work, thus giving the Allies many clues, known as cribs, which were major boosts in the code cracking process.

Beyond the improbability of being able to crack it in this manner, the strategies employed by the Allies to conceal the progress they were making was another major reason the Germans were kept in the dark. One such decision was made when the a British ship found and attacked U-110, a German U-boat. The ship surfaced after the crew had abandoned ship, believing it was sinking. They captured the surviving crew and brought them below decks. The British then boarded the ship and found a current version of an Enigma machine, which was important to the decrypting effort. By doing this, the crew themselves were unaware that their ship had been boarded, thus eliminating the possibility that German higher command would ever be notified that the machine had been captured, which would have led to an inevitable switch of encryption tactics.

Along with the secrecy surrounding the raiding of U-110, the Allies were careful about how they used the information they gained from intercepting and decoding Enigma messages. In one instance, a message was sent giving locations for a rendezvous for two U-boats, the purpose of which was to be resupplied by a mother U-boat. Two more U-boats were mentioned, including their plan to enter the Atlantic along with time and location. Instead of immediately attacking all 5 boats, the British took extra precautions to convince the Germans that the attack didn't happen because the messages were intercepted. They decided only to attack the three that were being supplied. Before attacking, they flew a spotter plane over the rendezvous location, which would give the Germans an explanation as to how the ships were found. After the Germans spotted the plane, a nearby battleship promptly sunk the three U-boats. Extra restraint such as this was essential to prevent suspicion that the Allies knew too much. Coincidentally, although the British military was not notified of the existence of the other two boats, an actual spotter plane happened to find them and both were sunk.

Although the sinking of 5 U-boats in one day would seem to tip of the Germans that their messages might have been intercepted and used against them, they never suspected that Enigma was compromised. Hitler did call a meeting with many of his generals and intelligence officials following the sinking of these 5 U-boats. Even though the possibility of Enigma being compromised was a major discussion point of the meeting, it was never fully considered, despite the racking evidence that it had been, which also included tips from Japan and Italy that were backed up by evidence. Ultimately, the confidence in the unbreakability of the Enigma code was the main reason that the Germans never found out that it had been broken. Despite evidence leading to that conclusion, they continued using the system all the way to the end of the war.

Looking back at the events of the war, many historians have predicted that if the code was not broken, the war would have dragged on for another two or more years. Events such as the invasion of Normandy would have been put off, and thousands more would have died. Some believe that the outcome of the war would have been unchanged whether or not the code had been broken, but we can safely thank the Germans, due to their faith in Enigma, that more deaths were prevented.

 
Sources:
"Crypto Wars: 2000 Years of Cipher Evolution" by Ralph Simpson
https://www.historynet.com/what-if-the-allies-had-not-broken-the-german-naval-code.htm
https://www.history.com/this-day-in-history/enigma-key-broken
http://ww2today.com/9th-may-1941-enigma-machine-captured
http://www.uboatarchive.net/U-110A/U-110.htm
http://militaryhistorynow.com/2014/11/24/cracked-military-historys-most-famous-codes-and-code-breakers