Ability to revoke/block own key

Would be nice to be able to broadcast messages like "this key was compromised" and/or "no further messages from this key are valid". This would address both the problem of private key already being compromised and add ability to lock the site forever (whether because there's a danger of key being compromised in the future or just because site is intended to be static).

When a node receives "compromised" message, it should stop accepting any further updates from the key and it would be nice to also mark data as potentially replaced by third-parties. When a node receives "no further updates" message, it does the same without marking data as compromised, and should also still react on "compromised" message (in order to prevent attacker to first replace content and then freeze it).

Potential drawback: this would add ability to kill all services based on private keys being public. But those are already susceptible to any kind of attack, so personally i don't think it's that bad. Also, it should be possible to also add "exception" feature if these are considered important enough.

PS: I think this was mentioned before on the network, but i couldn't find an issue here.

No files, tests, or entry points are named. Start by locating the protocol's key validation and update-acceptance paths, then determine how signed revocation messages would be represented and propagated. Done means compromised and no-further-updates messages reliably block later updates, preserve the stated distinction in data marking, and include tests for ordering and exceptions.

Vom Indexierungsmodell aus dem Issue-Text verfasst.