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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"?
Alexander Bolinsky
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  • What's the difference between "any" and "particular"? – Maarten Bodewes Sep 21 '15 at 19:25
  • 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. – Alexander Bolinsky Sep 21 '15 at 19:27
  • @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. – Maarten Bodewes Sep 21 '15 at 19:34
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    @MaartenBodewes AFAIU, duplicates aren't deleted automatically (unless unanswered and downvoted), so this would serve as a signpost. – otus Sep 21 '15 at 19:37
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    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 $p$. – Stephen Touset Sep 21 '15 at 19:53
  • Also, your clarification about the difference between "any" and "particular" only confuses the issue more. You say by "any" you don't care what the string is, and then you say by "particular" that there may be several, but you don't care what the string is — both of those seem to be the exact same thing. Additionally, it's unclear what you mean by the word "convey". – Stephen Touset Sep 21 '15 at 19:56
  • @StephenTouset, probably "any" = normal second preimage attack, "particular" = chosen prefix (or more specific format). But yes, clarification would be in order. – otus Sep 21 '15 at 20:24

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