I think you're right that it's technologically possible ... not even difficult, really. I do see some privacy issues -- how do you get that warrant approved?
"Yes, Your Honor, I'd like you to sign a warrant for unknown persons passing unknown, possibly copyright-infringing materials to other unknown persons.
"What's that? No, we don't know who any of these folks are. No we don't know what the materials are. No we don't even know where they are, so they may not be anywhere near your jurisdiction.
"How would we do this? Well we have to look at every file that comes through the intartubes and ....
I'm not sure that warrant would get signed, and I'm pretty sure it
shouldn't. Not in a free nation, anyway.
With child porn, there's actually a crime committed, so they may have better luck slipping that through, but copyright infringement is a civil matter, isn't it? (Regardless of what the RIAA might like us to think
) I can't see that getting much traction.
Its an interesting idea, and a very clever solution to the puzzle, but I don't think I like it.
