> So distribute keys on smart cards that don't allow you to export the key
That's what I covered in the second paragraph. :-)
The thing is, both those implementations were a disaster from either a technological or a security point of view. We're not even getting into whether a central source of trust is a good idea or not (you will look at the state of HTTPS and make up your own mind on that). So, to repeat, proper security is hard.
So distribute keys on smart cards that don't allow you to export the key. This is what Estonia does, and - concerns about their election infosec aside - it seems to work pretty well.