legal. You have the nuke but how are you gonna detonate it?

At this point, legal to disseminate. I won’t say this with absolute certainty but with reasonable certainty that the people/governments that have the technology, scientists, and finances to build an atomic weapon could if they wanted to because the information is available for the right price. But it’s complex, expensive, and unless you’re very good at hiding your endeavor, others at your level are going to notice where the rare and important minerals for enriching uranium are going. You can’t just dig up a rock containing uranium and build a bomb, but they’d notice if you suddenly took an interest in acquiring rocks that could be enriched, as well as notice if you were trying to get your hands on enriched uranium (or whichever rare source material you intended on using). I would doubt even the most structured, financed, and well organized terrorists, cartels, or warlords could build one even if they had detailed instructions, and no one’s building one in their basement in their spare time. Dirty bombs or someone getting their hands on a black market nuke (both very unlikely) would be something I would be more concerned about than someone building a nuke from scratch.

I’ll end on one example. The submarine billionaire who imploded himself. Schematics for deep sea subs exist, he had access to the kind of money that could have built one. He tried, he died. Now imagine that with nukes. Dude would have irradiated himself to death before he even started assembling it.

You need to teach people to the point that they understand what it is enough to avoid it. Otherwise they can be tricked into doing it or helping someone else do it. Knowledge is a barrier. Depending on your other options, you may or may not need it.

what is your threat model? the fact that one person can't plausibly know many advanced fields at once in sufficient detail limits risk significantly when you don't ignore that experts are rare

It should be legal and easy to do at no cost.

There is a slight difference between a weapon, and a nuke. One doesn't need a nuke to defend oneself and there is no 'what if', imho.

But if you insist that there was a 'what if', then I would tell you there would be no humanity left. We're already happily killing one another, using almost direct descent of stone age tools (a knife or a hammer), for the pettiest reasons (things like money or a bad look, come to mind) not even for imperative needs, like survival. So, the second the average people gets access to nuke power(even more today, seeing how poorly educated they're and how unable to handle the slightest emotion/contrariety), the entire planet will be blown away, say, because some neighbor dared listening to music too loud (or music one doesn't appreciate) or because their dog peed on their lawn.

"Yeah, but a single nuke won't blow the planet away!" Sure. But we will. One nuke after the other in as little time as it takes for enough of us to say 'they're the bad guy, they deserve to pay the price, where is my big red button?' (hint: we're all the bad guy for someone else, somewhere.)

End of humanity.

Yes. Historically cryptographic techniques have been classified as armaments and restricted on that basis.

We teach people to do all kinds of things. We also teach people why not-to do all kinds of things. They do them anyway.

Furthermore, if all that is needed is the information in order to be able to do the thing, theres no absolute way to prevent every person from accessing that information and, thereby having the means to do what they've be taught not-to do. So, in your scenario, someone, sometime, will use the weapon despite know not-to.

That said, wishing a nuclear weapon into existence — the highly-enriched uranium, the implosion core, the triggering mechanism, and the delivery system needed — seems like a helluva waste when literally everything else is also within reach. I'd hope that intelligence would be needed to understand the process; and that same intelligence would be sufficient to understand the possibility of a thing is not reason to do a thing. There's a very crude example coming up.

Personally, I think the command of matter-energy interconversion, higher-dimensions of reality, and cosmic-levels of power might inspire one to travel widely, live fearlessly, and create new possibilities. Petty grievances — those solvable with nuclear weapons — would seem like non-problems when literally everything else is not a problem either. No hunger, no cold, no lack of shelter, no distance too far, no need for authorities... just wish-fulfillment.

Summoning a nuke would be base, awkward to the point of being detrimental to all relations. It would be like a Supreme Court justice shitting in his own hands and deliberately smearing it on the faces of elders, kindergarteners, military service members, civil-rights activists, and puppies.

Like, ew. Who would do that?