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The title is pretty descriptive of my question. As an example, say I use the MD5 hash function to create a hash key of the word "hello": 5d41402abc4b2a76b9719d911017c592.

There are two questions I would like to have answered:

  1. How difficult would it be to calculate any new string that would hash to the same key as "hello"?

and

  1. How difficult would it be to calculate a particular new string that would hash to the same key as "hello"?
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  • $\begingroup$ What's the difference between "any" and "particular"? $\endgroup$
    – Maarten Bodewes ♦
    Sep 21, 2015 at 19:25
  • $\begingroup$ By "any" I mean I don't care what the string is, and by "particular" I mean there may be several strings that convey a particular thing, and I don't care what string it is, so long as it is a string that conveys what I want it to convey. If that helps. $\endgroup$
    – Alexander Bolinsky
    Sep 21, 2015 at 19:27
  • $\begingroup$ @otus It might be a good idea to clarify what a second preimage attacks is to the general public if we want that question to be a dupe. Otherwise people will never find it. $\endgroup$
    – Maarten Bodewes ♦
    Sep 21, 2015 at 19:34
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    $\begingroup$ @MaartenBodewes AFAIU, duplicates aren't deleted automatically (unless unanswered and downvoted), so this would serve as a signpost. $\endgroup$
    – otus
    Sep 21, 2015 at 19:37
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    $\begingroup$ Keep in mind that if you know $k$ where $k = H(p)$ and $p$ is low entropy (for instance, the string hello), it is not considered difficult to find $p$. $\endgroup$
    – Stephen Touset
    Sep 21, 2015 at 19:53

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