> IIUC the actual device that does the kaboom remained with its original design and applying voltage will be able to trigger it

That is not my understanding. My understanding is that the proper implosion requires very precise timing of signals for each shaped charge element otherwise the implosion ends up being lopsided and the nuke fizzles instead of exploding. These timings depend not just on the shape of the charges, but also on the relative wire lengths from the detonator to the explosives. (In theory these wire lengths can be unique for each warhead, thus making the timings for each warhead unique). The detonation circuit is not just comparing the code with an expected one, but using it to create the right signal timings. In other words the right code plus the information in the electronics together gives the timings for the signals with which they propagate through the different length of wires such that they form the right implosion.

To reverse engineer this you need to figure out when each explosive element needs to be triggered to form the explosion. Then you need to figure out when the signals need to leave the electronics such that it travels through the wiring looms just right to create the desired explosive pattern. And then you need to figure out what code you need to supply the electronics so it produces this desired electronic timing to achieve the above.

That is three wickedly hard challenge. And you will only know if your people pulled each of them off corectly, when you try to detonate the warhead.

> won't stop people with unrestricted access

That is true. But it is not like all they would need to do is to apply voltage on a single line, like some crazy hot-wiring car tief. Their best and easiest bet is to dissasemble the warhead and use the fissile material from it inside of an implosion device of their own design.

You may be right, the reason I assumed that the controller that controls the detonation itself was contained in the exclusion zone since earlier safety mechanisms were mechanical. So if they modernized the safety mechanism maybe they didn’t change the exploding part and all they need is power to prepare the device and then a simple signal to trigger it ?