I understand you don't trust the NSA, but the viability of what it takes to make a bomb is just basic well understood physics. There's no conspiracy there. You either have enough mass, at a higher enough level of enrichment and you can force them to go critical or you can't. For Uranium you need over 100lbs to build a basic gun type bomd, what's terrifying is how little Plutonium you need. The Fat Man bomb needed just over 6 kg of Pu. So a little less than 14 pounds, and that was with 1940s grade material. The USSR got pretty good at making good quality Pu.

The trick to keeping nuclear weapons out of the hands of someone is the security of the material, its hard to enrich. Its not hard to build the bomb. Enrichment is really really really hard. And its currently impossible to hide that your enriching. So you basically secure the material, and look out for people trying to make it. Thats how all non-proliferation works.

Nothings hard about the know how, everything you need to know is in the public domain. And dont forget, pure fission bomds are 1940s tech.

It's not a big secret how to build a simple gun type bomb. You don't need much precision for that. You get enough HEU, and it's game on. You get the right shaped Pu and a few other items, and you have an even higher yield bomb than that of the HEU bomb. Again, 1940s technology is all you need.

Fusion bombs, yes much harder. But who needs that when you can build a Nagasaki yield bomb?