From: Chris Doherty (chris-freebsdrandomcamel.net)
Date: Wed Aug 18 2004 - 15:54:41 CDT

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On Wed, Aug 18, 2004 at 02:29:57PM -0400, Peter C. Lai said:
> On Wed, Aug 18, 2004 at 09:08:12PM +0300, Claudiu wrote:
> > hello,
> >
> > please explain what do you mean by "reverse the hash". Is this the
> > recreation of the originial message from its hash ?
>
> The short answer is yes. The slightly longer answer is that such is only one
> specific case. The general case is that the digest should not reveal any
> information about the original message.

well, technically you're not "reversing the hash": you can't re-create a
message from its hash, because the information is simply gone--digesting
algorithms are massively lossy by definition. that is, you can't take a
128-bit MD5 hash and recover the original 2-megabyte message, which makes
sense.

what you can do, if you have a proper attack formula, is find *a* message
that produces *that one hash*. that is, if I have message M which produces
hash H, I can use the attack to find *a* message M' which will also
produce hash H.

I suppose the possibility exists that M' will equal the original M, but
I'd speculate that the odds are remarkably small.

chris

-------------------------------
Chris Doherty
chris [at] randomcamel.net

"I think," said Christopher Robin, "that we ought to eat
all our provisions now, so we won't have so much to carry."
               -- A. A. Milne
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