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73

Short Answer Allied superiority in cryptography versus both the Germans and the Japanese can be broadly attributed to (1) better/greater coordination among personnel, awareness of vulnerabilities, and allocation of resources for breaking enemy codes and, (2) the fact that Axis codes were (mostly, though not always) more easy to break than Allied ones. ...


71

During that period, both Churchill and Roosevelt were old men more used to hand written letters than "high technology" teletype writers. Teletype writers are NOISY! Using teletype writers can be a slow process & thus make a l-o-n-g conversation. Telephones, despite sophisticated encryption technology, are immediate and more intimate. In ...


53

I nominate the breaking of the code generated by the German Enigma machine during World War II by British cryptographers. Hundreds, if not thousands, of people were involved with the project which was ongoing throughout the war. The secret was not revealed until many years after the war ended. The Germans never knew that their codes were broken.


47

To answer why they denied its existence, because the value of British code breaking relied on keeping their ability to break codes secret. (Sorry this is without sources, I'm on a phone on a train.) Part of the wild success of British code breaking during the war was due to the Germans never realizing their communications were compromised. The Germans ...


46

One man's lock is another man's puzzle. Combination locks have been used since at least ancient Rome. Whether the lock uses numbers or letters (or other symbols), the combination to be entered may be set based on a riddle or some other piece of knowledge as a mnemonic. The lock is meant to be solved at some future time by someone who has the correct ...


42

The Copper Scroll The Copper Scroll is a Dead Sea scroll found in 1952, unique in that it is of copper (with a little tin), has a list of 63 or 64 locations of treasure with "obscure hints of the locations". Although it was initially disputed whether or not the list was historical rather than legendary, a scholarly consensus seems to be emerging that ...


23

We can't know the biggest successful secret project, just big revealed ones. Here are some examples with 5, 10, 20 years duration. Project Azorian ran for about 5 years from start to disclosure. 160 on the ship, countless more on shore. The F-117. Contracted in 1978, except that the 1xx designator fits into a much earlier era. The existence was officially ...


17

To understand why the Ultra secret had to be kept secret, one has to look at the encryption technology that was in use during the post war period, and in many cases is still in use today. Although we may consider the German Enigma machines just technology from the second world war, most people never imagined that that technology continued to be in use for ...


14

For a print work I recommend Francis L. Loewenheim, Harold D. Langley, & Manfred Jonas, Editors; Roosevelt and Churchill: Their Secret Wartime Correspondence, Dutton, 1975. Contains some 600 or so of the more than 1700 cable messages which passed between Roosevelt and Churchill from shortly after the start of the war in 1939 until April 1945. Or if you ...


10

From 1950 to 1960, "Psychic driving" was a CIA program related to MKUltra which involved medical tests erasing people's minds using drugs and electricity and reprogramming them with tape recordings played thousands of times. https://en.wikipedia.org/wiki/Psychic_driving Operation Legacy ran from the 50's to the 70's and it involved erasing all of the ...


5

The Egyptian labyrinth(s) could possibly be an example of actual treasure hidden behind a puzzle. I had a little difficulty finding a source that "felt reliable". This tantalizingly detailed description: You entered the maze from a descending stairway, hidden on the south side of the pyramid, which led to a small chamber. This apparently led nowhere;...


3

A very large successful secret project was the violation/revision of the Versailles treaty by Germany, over the 1920s and early 1930s. It began no later than about 1922, with a German-Russian "understanding", following the formal Treaty of Rapallo. At that time, Germany tested tactics using such "forbidden" weapons as tanks and airplanes on Soviet soil, ...


2

Just to give a concise, general answer: the destruction of the documents was obvious (“of course”) to your author because the end of a single war does not mean the end of all wars. The British government had to be alert to the possibility of another war, perhaps quite soon (a possibility that was not unthinkable). To maintain their advantage, no matter what ...


1

The Deceivers by Thaddeus Holt is a history of Allied deception operations in WWII, but it necessarily also covers codebreaking. I'm not sure how objective the book is (it seems to be rehashing some British/American feuds) but it was quite dismissive of Japanese intelligence operations in general. Lots of Allied (British) effort to plant false clues and then ...


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