Although Singh argues that the primary reason that the Allies had success over the Germans in the cryptographic war, I believe that this simplifies the argument way too much. While undoubtedly the Germans were overconfident in the security of the Enigma machine, this was only a problem when they became lazy and began to repeat messages, giving the Allied cryptanalysts a chance to break their codes.
Perhaps one of the most overlooked parts of the Allied codebreaking success was the determination and resilience of the code-breakers, and on top of that how diverse they were. As a group consisting of people from so many different backgrounds, their different ways of approaching the deciphering were no doubt crucial in the Allies breaking the German codes. Furthermore, it can not be overstated how impressive the resilience of the codebreakers was. Most days they worked fruitlessly for hours upon hours in an attempt to crack the codes and got absolutely nowhere. And then as soon as the clock struck midnight all of their work from the day before was rendered useless and they had to start all over again. While this would drive most people mad, the Allied cryptanalysts continued to decipher day after day.
Finally, the Allied codes were so strong because of the rarity of the Navajo language. Trying to understand a language without any indication of what any words mean is nearly impossible and the Germans were certainly among those who discovered this. Furthermore, when they combined the language with code words it became impossible for the Germans to break it without capturing an actual Navajo who would be able to decipher the messages for them. This brilliant way to securely transmit messages for the Allies proved to be a crucial part in them winning the war.