Imagine the task of keeping the D-Day landings a secret. It would have been impossible to ensure 'absolute' security with the millions of troops stationed in Britain and all the construction and planes and equipment. the blokes responsible for the 'Pattons Dummy Army' with false radio traffic and all. :o
I think they did a magnificent job at keeping the Germans, and even the Allied men guessing!! 8)
Normally, in such a huge operation, it's standard practice not so much to 'blackout' all information but rather to control certain peices of 'misinformation' and to plant false leads etc. This way, at least, if the enemy does get lucky enough to stumble onto something, they won't know if it's genuine or not. ???
That was the problem with making the "Man with no Name" (Major Martin, who was really an unfortunate vagrant who drowned in the Thames) plan a success. Convincing the Germans that the information was genuine! ;D