I'm not a lawyer either, but it would seem Signal would have less culpability operating a network of users knowing that they do not have any access to the conte

I'm not a lawyer either, but it would seem Signal would have less culpability operating a network of users knowing that they do not have any access to the contents of the messages and thus have an implicit impetus to do nothing. Then again, they could be held in contempt of court (maybe?) because sticking your fingers in your ears and shouting La La La isn't really a good defense for ignorance of a fact, if they were to be given evidence that illegal pornography was being transmitted through their servers. Basically, it seems like it probably would have less effect on them than say Tumblr, which does in fact host a lot of pornographic material in the clear, but they could still be thrown into the mix depending on circumstances.