Comment by jdougan

It seems like aspects of miniaturizing multi-stage fusion devices can still be difficult. Single stage unboosted fission devices should be be near trivial, so long as you have a source of fissile material (as you indicated).

The other secret-ish aspect is details of the safety features and permissive action links. I say -ish because at one point the US offered to give PAL technology to the Soviets.