AFAIK the Germans kept improving both the Enigma itself and the procedures for using it. For example there was some specific weakness that the Polish attacks us

AFAIK the Germans kept improving both the Enigma itself and the procedures for using it. For example there was some specific weakness that the Polish attacks used, but at some point Germans figured it out and "patched" it. By the end of the war they could change the "encryption key" even more often than once a day, whereas before the war they could reuse the same key for many months. The Poles didn't break all Enigma setups till 1939 because one didn't have to it yet to effectively collect intelligence. Also because they hadn't existed yet :) Enigma was actually pretty good, but it had a major design flaw: insufficient diffusion. The YouTube channel Computerphile made a good video on practical cryptanalysis of the Enigma using modern hardware: https://youtu.be/RzWB5jL5RX0 https://youtu.be/RzWB5jL5RX0. In short, if you know what you're doing you can do ciphertext-only attacks given long enough messages and minimal knowledge about the distribution of the plaintext (their example relies only on the fact that the plaintext is in English).