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.

There is nothing interesting is the design itself, the hard part is how to produce some very intricate parts and materials.