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In the scene from The Imitation Game, where they are able to crack engima, Alan convinces his teammates to not prevent the next attack.

He says something like:

Lie to somebody when they are expecting to be lied.

What does the above line mean?

And how is preventing this particular attack riskier than preventing any future attack?

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How is preventing this particular attack riskier than preventing any future attack?

The answer is given at 2:05 in the video. If the British suddenly attack the U-boat:

"The Germans will know we have broken Enigma."

"They'll stop all radio communications by mid-day, and they will change the design of Enigma by week-end."

The Germans will quickly realize there was no other way that the British could have learned about the attack. They will realize that the British must have cracked the secret Enigma code.

The Germans would then cease all radio communications, and change Enigma within days. If they do that, the British will lose this incredible military advantage that they have. It could take years before they could crack the upgraded version of Enigma, and there's no guarantee that they actually could crack it. The value of being able to decipher the coded German messages, and thereby having an advantage in every future battle and probably the war, outweighs the value of preventing this one attack. It's tragic but logical.

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  • I think this bypasses the question... you say that they'd rather have an advantage in future battles, but the point of the OP is that eventually when they use the intelligence to their advantage, they risk the German's discovering the truth. So if they aren't willing to use it in this case; what will make them more willing to use it in any future case? In other words, the argument given by the film (and by you) can be equally applied to any situation in which they are considering using information that they learned from Enigma. – GendoIkari Oct 17 '19 at 17:06
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    I see what you're saying, and I'm no military strategist, but for future battles they can plant disinformation so the Germans think there are plausible alternatives to explain how the Allies won the battles. For example, the Allies might have broken the Enigma code, or the Allies might have gotten the information from a high-ranking German prisoner. Since the Germans won't know for sure, the Germans won't change the Enigma machine. In fact, I think that may have been discussed later in the movie. Anyway, to your point, Allies could use Enigma information but they had to be clever about it. – BrettFromLA Oct 17 '19 at 18:30
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I presume this scene is based on a famous urban legend from WWII, known as the "Coventry dilemma".

The story goes that, in November 1940, Bletchley Park decoded an Enigma-encrypted transmission revealing that a bombing raid on Coventry was planned for the 14th. The dilemma was thus: did the British prevent the raid, and risk revealing to the Germans that they'd cracked the Enigma code, or do nothing and keep the decryption a secret at the cost of hundreds of lives?

Winston Churchill supposedly chose the latter, and let the raid happen. 568 people were killed, and central Coventry was almost totally destroyed, but the Germans were left unaware that the Enigma had been deciphered and Bletchley Park was able to go on intercepting German communications.

This is why Turing is reluctant to act on the information they've learned from having cracked the Enigma code: if they do, the Germans will realise the Enigma has been cracked, and change their encryption method. By doing nothing, and pretending they haven't cracked the code, they are effectively lying to the Germans about how much intelligence they have.

[For the record, there is no evidence that the British actually knew of the Coventry raid in advance. It's merely an urban legend. See Wikipedia's section on the myth for more detail.]

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    Amusingly enough, there's a scene in the BBC's Sherlock where Sherlock (also played by Benedict Cumberbatch) delivers a monologue about the Coventry dilemma. I'll have to find it when I get home, so we can have Benedict Cumberbatch explaining Benedict Cumberbatch's actions. – F1Krazy Oct 17 '19 at 12:34
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    It is worth mentioning that although "The Coventry Dilemma" may well have never happened, the reverse most certainly did - the allies deliberately chose to NOT attack vulnerable german units (ie, not notifying local commanders) . On one occasion IIRC, they had an aircraft carrier send planes to attack some shipping but chose to not inform them about a couple of U-Boats. In the event, some of the returning planes happened to spot and sink them anyway, but that did lead some German commanders to raise questions. Fortunately the questions faded away, but shows how close they judged it ! – racraman Oct 18 '19 at 3:35
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To add to the other two answers, shortly after this scene in the movie they discuss determining HOW MUCH they can interfere, and how to interfere with German plans, to hide the knowledge that Enigma is broken. The fact that Enigma was broken was kept a secret from almost everyone, with Alan's team leaking out what THEY chose for the army to act on and how to act on it.

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