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This question (update: now closed as duplicate) asks a basic thing: why the security level of 2DES is 57-bit (implicitly: given a few plaintext/ciphertext pairs). A helpful (if not quite correct) answer was given. Update 2: that answer was fixed.

The closer thing to a duplicate question seems to be this, asking why the meet-in-the-middle attack is considered to succeed in a maximum number of 2n+1 encryption operations for an n-bit key cipher, rather than 2n+2 for a simple implementation; with DES given as example. There's a good answer to that other question.

I hesitated between

  • Voting to close the question as a duplicate. Update: I'm ready to bet that the OP will have a hard time figuring out things from a link to a more complex question with a detailed answer about a technicality of meet-in-the-middle attack, and two links to questions considering 3DES. The answer , although not quite correct, is probably the most helpful one, but is going to disappear in oblivion if the question remains closed.
  • Answering the question, and then
    • doing so with another answer,
    • or fixing the existing answer myself until it contains no false assertion,
    • or (as I did) pointing in comment the logical error in the answer, hoping the answer to be fixed (which happened, but only after the question was closed, which is a timed death sentence for the answer).

What was the best action? Should the question be re-opened?

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    $\begingroup$ To solve the problem of too complex answers was my goal: My answer is (in my opinion) understandable by people not familiar with complex cryptographic matters, but enough to understand the basics of the questions. Other linked answers can give a deeper view to the technical details. $\endgroup$ – Nova Apr 19 '15 at 11:42
  • $\begingroup$ @Nova: indeed your answer is simple, and the most helpful, and now fits my sense of rightness. That's basically why I have a problem with closing the question and loosing it. $\endgroup$ – fgrieu Apr 19 '15 at 14:05
  • $\begingroup$ Should "An helpful" be replaced with "A helpful"? $\endgroup$ – user991 Apr 29 '15 at 3:00
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First off, we won't be losing the question linked to above. Duplicates are not automatically deleted, they are automatically deleted if they have been downvoted.

As far as whether or not it should be closed, I think that depend on whether you view it as an accidental duplicate or a borderline duplicate. Not all "duplicates" are bad.

So, what should be done? Well, I think you are doing the exact right thing. Raising awareness via comments on the question and here on meta. This should encourage others to vote to reopen if they feel that the "close as duplicate" was incorrect. Given enough reopen votes, it will be reopened.

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  • $\begingroup$ Ah, did no know that only downvoted duplicates vanish over time. That's nice ! $\endgroup$ – fgrieu Apr 20 '15 at 14:34
  • $\begingroup$ Reopen vote cast. We'll see if others agree. :) I do agree that this one's a borderline case; I'd rather see it reopened, but if others think it's better off closed, that's fine with me too. $\endgroup$ – Ilmari Karonen Apr 30 '15 at 17:34

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