In addition, some of the apparent operation and security errors that helped made Enigma signals readable by the Allies were likely the deliberate work of other anti-Nazi German officers. A substitute for Enigma would have been vulnerable to such honorable sabotage and errors in signals security.
Moreover, a mere change in machines would not have transcended the fundamental limitations of German code technology and practices or counteracted the scope and sophistication of Allied code breaking efforts. Unrealized by the Germans, the Allies had attained analytical and computational advances that far outpaced Germany;s ability to develop and adopt secure code systems.
In the end, whether Nazi Germany used Enigma or some substitute machine, the Allies would probably have read German's wartime signals.
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I think you are overstating the case there.
How did this ‘Fellgiebel’ even know that the Enigma had in fact been compromised?
Furthermore there were three separate inquiries into the the cryptographic security of the Enigma(to see if the German secret communiques had been breached, which many Nazis had suspected) and concluded that the Enigma was not at fault.
The British did not invent the Computer-—Colossus was not a computer. Just a pile of digtal circuitry dedicated to one purpose: Juxtapositioning of Lorenz code characters.
Finally I think you are grossly overstating the magnitude and effectability of the German Resistance.
There are many many reasons why the Germans/Nazis wholly adopted the Enigma to the exclusion of all else(Lorenz Cipher machines were only used by the German Army High Command)
Go read David Kahn’s book. It is a good read and very detailed.
From the publisher's description:
For the last 20 years books on the Allied effort in World War II have placed great emphasis on the fact that the Allies had broken the German "Enigma" codes and often had advance warning of German activities. For some years, knowledgeable researchers have felt that a counter-argument needed to be made, that perhaps undue emphasis had been placed on Allied code-breaking successes.
Kenneth Macksey here makes just such a counter-argument. A fresh study of the evidence leads Macksey to argue that anti-Hitler generals knew that Enigma had been broken and were playing a sophisticated double game to bring down the Nazi regime. Particular attention is given to the activities of General Erich Fellgiebel, head of German Army Signals, who was executed in the wake of the Hitler assassination plot.
How significant a contribution did the German anti-Hitler officers make toward Allied victory? Much of the evidence escapes the historical record or was suppressed. Even today, key files are still withheld under Britain's 75 year rule.
Here is one illustrative mystery. When, just before D-Day, the Germans issued the invasion alert for all of the north of France, Normandy was unaccountably left out, thus delaying the German response to the invasion by a few critical hours. Why? Could it have been because of anti-Nazi German Army officers? In the absence of other evidence, that seems the best explanation.
Similarly, there are instances in which Allied code breaking was facilitated by mistakes in Enigma security practices. Sometimes, the same message would be transmitted using both old and new code wheel and plug settings, or such settings would even be transmitted in violation of strict security protocols. Bone headed mistakes or sabotage? In the absence of proof, we are left to speculate.