Re: Infecting the KaZaA network? (moving here thread from 'traq)

["Shoten" <shoten () starpower net>]

> > It is quite possible given two plaintexts of sufficient size, to ensure
> > that they both have the same MD5 checksum.
>
> I think this is significantly harder than you are making
> it out to be. MD5 is a cryptographic checksum designed specifically
> to resist this kind of "collision".
>
> MD5 is weaker than SHA-1, but it is not so weak that you can just
> go around forging it at will.
>
> It would take about 2^64 hashes of random inputs to find two
> distinct inputs with the same MD5 (a collision).
> This is due to the birthday paradox and the 128 bit output of MD5.
> Note that if you want to collide with a specific MD5 value
> (as you would need to do to mount the infection you are talking about),
> this problem is much harder.
>
> In that case, you end up having to do more like 2^127 hashes
> of random inputs to find another input that hashes to the
> same MD5 as a specific fixed input (the file your are spoofing).
>
> Don't hold your breath waiting for 2^127 hashes to finish.

Not to mention that in this case, the file with the same checksum would have
to be EXACTLY the same size as the KaZaA executable, AND be a functional
virus on top of that.  And even if you got all that, you'd have to worry
about it getting mixed with a valid client during download from multiple
sources.  For those who think this is possible, go ahead and try...good luck
:)