Reinforcing what I said about nuclear weapons secrecy actually keeping information out of the hands of those who would use it badly (that includes both terrorists and semi-competent amateurs playing at WMD), Alex Wellerstein examines The Nth Country Experiment. The Livermore Laboratory employed a couple of recent Ph.D.s and told them to design a bomb from open-source materials. His conclusion, through a heavily-redacted version of the report, is that there's some question as to the potential effectiveness of the design they came up with.
That was in 1967, long before the internet. I see the claim frequently that "all the information is out there," but I have my doubts about that. And, as Wellerstein points out, the design is only the first step.
I won’t even bother pointing out that “designing” an atomic bomb on paper is, of course, nothing doing compared to actually producing the fissile material, casting explosives, fabricating the right shapes of things, assembling the whole device, all the while not killing yourself in the process. Headlines aside, these guys did not build an atomic bomb in any sense of the term “build,” which I think most thoughtful nuclear observers realize.Much of that information is classified too.