Nuke must be armed and the correct firing sequence is required for correct detonation. If you blow up a nuke you just scatter the fuel. Often, to get the correct detonation the fuel is brought together in two or more parts and then an exploding casing is used to crush the fuel into a small space, the casing is arranged so there are segments that have different reaction times to a signal, so the signal must be a sequence in the correct order and timing that only the genuine firing mechanism can deliver - something "like" that.