What if I wanted to use Dropbox for piracy purposes (I don't, just using an easy to understand model). Could I upload a copy of Avatar.mp4, compute or find the checksum for it, and then distribute just that checksum to other users? Or distribute a file that wasn't Avatar.mp4, but looked like it to the Dropbox client? Would this effectively trick the Dropbox servers into distributing the file for more, so that I could legitimately claim to not be distributing copyrighted material, but just distributing a "magic" file that caused Dropbox to give me a copyrighted file that someone else had uploaded?
Or distribute a file that wasn't Avatar.mp4, but looked like it to the Dropbox client?
Modern hashing algorithms make this practically impossible, sorry.
Just because you have a hash string that's not technically a copyrighted film, you could still be guilty of copyright infringement. cf. http://ansuz.sooke.bc.ca/entry/23
If they're using a decent hash algo, manufacturing a collision should be somewhat difficult and I'm really not sure how you could use it as a believable legal defense later.
What if I wanted to use Dropbox for piracy purposes (I don't, just using an easy to understand model). Could I upload a copy of Avatar.mp4, compute or find the checksum for it, and then distribute just that checksum to other users? Or distribute a file that wasn't Avatar.mp4, but looked like it to the Dropbox client? Would this effectively trick the Dropbox servers into distributing the file for more, so that I could legitimately claim to not be distributing copyrighted material, but just distributing a "magic" file that caused Dropbox to give me a copyrighted file that someone else had uploaded?