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I've seen from a number of sources that both the Colossus and the cryptological bombes operating for England were dismantled after the war ended. The Wikipedia article even says that all of Colossus' documentation was burnt.

I would guess that at that time it would be clear to anyone that the need for cryptanalysis would not end with the end of the war. What is the rationale behind this?

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    $\begingroup$ I would assume there was a need for secrecy. Germany was no longer in a huge position of power, but Churchill, for one, never trusted Stalin to the degree that Roosevelt/Truman did, and there were likely others in the British government who agreed with Churchill. $\endgroup$ – HDE 226868 Sep 26 '15 at 23:49
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    $\begingroup$ Another reason may be: Britain was practically bankrupt, but "Colossus and the cryptological bombes" could be scrapped and sold for money. $\endgroup$ – Gerald Edgar May 7 '19 at 23:08
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  1. To keep secret the level of expertise in cryptanalysis so future opponents wouldn't put effort into improving their own codes. Probably pointless because it was inevitable that some details of Bletchley would leak to the USSR, in the same way as secrets of the Manhattan project. And probably equally pointless in that, although every side in WWII broke almost all of the opponents codes - they all believed their own codes were unbreakable so put little effort in counter-crypt-analysis.

  2. The UK government gave captured Enigma machines to their allies - claiming that they were unbreakable.

  3. Allied leaders wanted the glory of people believing that they had won through their own bravery and military genius. Not because they had been reading all the opposition's orders before they did.

  4. Standard government procedure to keep everything secret until compelled otherwise.

  5. Finally and possibly the most important: To not give the losers a simple excuse for why they lost. If the message becomes: Our invincible army only lost because of poor crypto then we would win if we fixed this problem and and so should try again.

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