There is reasonable evidence that ICE is already tracking protestors using facial-recognition technology.[1] Since ICE already has a lot of information about protestors and it is very difficult to hide information from agents, posting photos on social media makes little difference to most people’s safety, while simultaneously encouraging and informing others about the protests. Of course, there are various ways to make it difficult for ICE to identify someone, but most people are not going to do that.

A Layer-3 (network-layer) blacklist risks cutting off innocent CGNAT and cloud users. What you’re proposing is similar to mechanisms that already exist (e.g., access control lists at the ISP level work by asking computer B which requests it wants to reject and rejecting those that originate from computer A). However, implementing any large-scale blocking effort beyond the endpoint (i.e. telling an unrelated computer C to blackhole all requests from computer A to computer B) would be too computationally expensive for a use case as wide and as precise as “every computer on the Internet”.

Also, in your post you mentioned, “A host would need to have a way to identify itself as authoritative, responsible for the IP address in question.” This already happens in the form of BGP though it doesn’t provide cryptographic proof of ownership unless additional mechanisms are in use (RPKI/ROA).

Planners do a lot of preparatory work before laying cables, but based on the few articles and studies I could find online, it appears they rarely share their findings with the public.

Sabotage can also be carried out without submarines. For example, a ship could drag its anchor along the seabed (whether in the Baltic Sea or the Taiwan Strait) near known locations of internet cables.