[blocko]

I think the solution would be sending as little data as possible to answer the query, and choosing which/how many queries to respond to. Any large scale requests for data would have to be designed in a way that minimizes intrusiveness or risks being largely ignored. I don't think there's a great way to counter spoofing or sending garbage results without sacrificing user control, but the onus to de-noise should be on whatever institution is attempting to collate data. Additionally, if it's an opt-in system focused on public good there would generally not be much incentive to mess with it as long as users are somehow verified as being in the community (cert generated/granted once after initial identity check, etc). Sadly this is all very pie in the sky at the moment, the amount of cultural change + added infrastructure required for any system like this at the scale of a country would be insane

[thinkmassive]

That's a good point. A common example is how a bar doesn't need to know your birthday, let alone your home address or driver license number, but right now it's common to hand over your entire id. All they need is a yes/no answer to "is this person 21+ today?"