Please excuse me if I don't use proper terminology, I'm a lawyer and trying to get more information about something my son has pointed out to me.
In Germany, law enforcement agencies use enhanced versions of Emule to search for kiddyporn uploaders. Basically, they're looking for the IPs of people offering files having file sizes and hashes that correspond to known illegal movies, and if they find those, they have the provider match the IP address to the customer to conduct house searches etc. They say the probability of a different file having the same hash is very much lower than the probability of two fingerprints being identical, so it's unnecessary to download the whole file and check if it is indeed the correct one.
(Please note that this alone isn't sufficient for a conviction if the house search results in nothing, but the search, seizure of computers and other hardware etc. along with the psychological pressure is still not something you'd want to experience.)
When I discussed this with my son to understand the details, he told me this line of thought was flawed. He said that Emule uses MD4 hashes, which are considered insecure. So, to stay in the fingerprint analogy, just like you can craft a latex glove having the fingerprints you want/need, you could easily take a harmless file (think cute cat picture), inflate its size in a way that the additional bytes cause it to have a specific MD4 hash, give it to the neighbor you hate, and if the neighbor shares that file on Emule, he'd be in for a very nasty surprise.
Googling for "MD4 insecure" yields a lot of answers which I honestly don't understand, that's why I'm asking here.
- Is it possible at all to create a fake file in the way described above?
- Are there tools out there that would allow a non-technical person to create such a file without much effort?
- If there are no such tools, how long would it take a dedicated, skilled person to create one?
- Are there any records that this has, ever, been attempted, successfully or unsuccessfully?