In the period 1941-44 the cryptanalysts of the German army’s codebreaking department Inspectorate 7/VI investigated the security of the Enigma cipher machine.
I’ve copied the relevant passages from the War Diary of Inspectorate 7/VI and as soon as I get some accurate translations I’ll post the text.
Unfortunately I don’t have the files of the period 1939-40. However it is clear from the TICOM reports DF-190 that the double encipherment of the indicator was identified as a security weakness and that’s why it was changed in 1940.
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