The law simply states it's illegal for a felon to vote... they forgave that right when they did whatever they did. So... how hard is it to have a national database of everyone in this country that listed where you were registered and if you were a felon or not? Is it really that complicated?
So the hard part... where do you get a database of everyone who's eligible to vote? Oh I dunno, maybe the Selective Service one we're required by law to register for when we turn 18? And I'm sure there's a database in each state, if not the FBI, that lists everyone who's been in jail or is in jail? Seems like an easy task.