[dmurray]

The last one sounds like this: A zero-knowledge protocol for nuclear warhead verification [0].

Sadly, I don't think this is actually helpful for nuclear arms control. I suppose you could imagine a case where a country is known to have enough nuclear material for exactly X warheads, hasn't acquired more, and it could prove to an inspector that all of the material is still inside the same devices it was in at the last inspection. But most weapons development happens by building new bombs, not repurposing old ones, and most countries don't have exactly X bombs, they have either 0 or so many the armed forces can't reliably count them.

[0] https://www.nature.com/articles/nature13457

[mdhb]

I don’t think this is actually the one I had in mind but it’s an interesting concept all the same. Thanks for the link.