It seems like every country that has been "allowed" to use nuclear weapons has figured it out though. It isn't like there are any that set off on this course and failed. AFAIK they all pretty much succeeded except Iran, probably because of all the blowing up of enrichment facilities. South Africa pulled it off. Israel pulled it off. North Korea pulled it off. India and Pakistan both pulled it off. Seems like anyone can do it if allowed to be pursued. France and England pulled it off. Canada too. What is "assumed" about the design in public knowledge seems pretty much solved in all but the exact nuance of how the secondary is triggered via gamma or xray, going off the Wikipedia article at least:

"The crucial detail of how the X-rays create the pressure is the main remaining disputed point in the unclassified press."

Then the article goes on to list the three leading theories. This seems like something you can probably evaluate for sure with a few bomb tests, again, if allowed by the controller of the planet, the USA.