I don't completely understand how demonstrating the chain of investigation should require the disclosure of the vulnerability. I really hope the execution of the law isn't going to depend on 12 random people understanding the finer points of cryptanalysis.

I wish there were some way to get a private but independent verification that X technique allowed IP addresses to be collected if they did action Y (e.g. logging onto playpen). Then the investigation could be questioned without the government disclosing their vulnerability, which seems like a standard we wouldn't hold most to. If someone were on trial for copyright infringement could they subponea for the full source of FairPlay as a coercion to get the case dropped?