The Pointlessness of the MD5 "attacks"

[Anton Stiglic]

>David Wagner wrote:
>> Ben Laurie writes:
>
>
>> Or, even more contrived, imagine that img1.jpg looks
>> like a completely normal JPG file, but img2.jpg exploits some buffer
>> overrun in the startup screen's JPG decoder to overwrite the program's
>> image with some other malicious code.
>> 
>> Sure, these scenarios are contrived and unlikely.  But how do you
>> know that there is not some other (possibly more complex but less
>> contrived) scenario that you would consider more troubling?
>
>They do not relate to the known MD5 collisions - these are general 
>collisions, which we do not know how to create, not the restricted ones 
>we do know how to create.

I disagree; I think it might be possible with the current cryptanalysis on
MD5.  The collisions that can be currently produced only flip a couple of
bits, and you can add what you want before and after the 1024-bit block.

Imagine some code that reads the image (or whatever bit-string) as a textual
string, in one case it doesn't read the whole bit-array because there is a
null string-terminating character, in the other case (collision) the
character is not present and causes a buffer overflow.  I think something
like that can be done today.

--Anton




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