They don't have to block every user using unlawful encryption -- they just have to have a law that lets the penalty be enough that a few high profile conviction

They don't have to block every user using unlawful encryption -- they just have to have a law that lets the penalty be enough that a few high profile convictions will deter enough law-abiding people that it isn't worth the risk. At that point, they can simply listen to all traffic and, if there are packets that aren't listenable due to encryption, assume ill intent, then be free to react appropriately. Assuming high enough signal ratios in detecting encryption, it might actually prove that encoded messages, or steganography proves to be more effective at preventing state-level eavesdropping... they won't prevent a determined state from getting your secret content, but it could be enough that, if the messages looked unencrypted, they would be passed over for the more obviously encrypted messages -- hiding in plain sight, as it were. Note, this is not an endorsement of any privacy invasions whatsoever, but musings on what a post-4th-amendment-obliteration world might look like for the privacy-minded.