Talk:Key strengthening

From Wikipedia, the free encyclopedia

This article is part of WikiProject Cryptography, an attempt to build a comprehensive and detailed guide to cryptography in the Wikipedia. If you would like to participate, you can choose to edit the article attached to this page, or visit the project page, where you can join the project and see a list of open tasks.

[edit] Worth 72 bits?

Another way to think of it is that 65000 rounds in the loop means about 2¹⁶ operations, which means the stronger key is "worth" about 16 bits more in key strength. If the weak key is a 56-bit "export key" then after key strengthening it is worth 56+16 = 72 bits.

This part seems just wrong, since the strength of a 56-bit key is only worth 56-bit when attacked with a brute force exhaustive search. If you use a dictionary attack then a real password usually isn't as strong as the derived key could potentially be (in this case 56-bit). Therefore it's not the output key that is strengthened but the input, which is much weaker.

In other words: you can't exceed the strength of the output key, only the weak input.

If you'd try to strengthen a totally random 56-bit key by repeatedly hashing it and then cutting it down to 56-bit again the cost of an attack wouldn't change, since you could just perform an exhaustive search on the output key instead of the input.

Thus i'll remove that paragraph.

--89.55.230.143 17:18, 7 March 2007 (UTC)

Well, you missunderstood that paragraph. You are right that "You can't exceed the strength of the output key, only the weak input." However, the output key is a "stronger key" and should be the full result of the key strengthening. That is, 128 bits or more. We should NOT cut the output key down to 56-bits after strengthening it. The export laws (thankfully) do not require that.

I put the paragraph back into the text and changed "it" in the sentence to say "the stronger key" to make the paragraph clearer. So the sentence is now: "If the weak key is a 56-bit "export key" then after key strengthening the stronger key is worth 56+16 = 72 bits."

We can extend the paragraph to explain that the output stronger key should NOT be cut down to 56 bits again but should be kept as 128 bits or more. If you find it necessary to make it understandable?

--David Göthberg 11:44, 4 April 2007 (UTC)