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MD5 and SHA cracked/broken...

David Bolen db3l at fitlinxx.com
Thu Sep 16 11:12:03 EDT 2004 
Paul Rubin <http://phr.cx@NOSPAM.invalid> writes:

> You don't need preimages to plant a trojan.  If you can create mere
> collisions, you can create two files, one with a trojan and one
> without a trojan, that have the same md5sum.  You publish the
> non-trojan one, people inspect it carefully and start using it, and
> download sites say that its md5sum should be so-and-so.  Now you can
> replace the non-trojan file with the trojan version and the md5sum
> will still verify.

But why bother?  Clearly in this case I'm in control of the md5sum's
publication (since I'm formulating both files to match), so why
wouldn't I just publish the trojan one in the first place with an MD5
that matches the trojan?  Any user of my package is already trusting
that any MD5 I publish is in fact for a proper file, so they are in
effect already trusting me.

I'd be more concerned that another party (other than myself) was able
to insert a different file that matched my original MD5 that I had
published.  It sounds like this exploit doesn't impact that ability at
this point.

-- David
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