I had an idea of assigning each bubble on a paper ballot a unique prime number. You could fill out your sample ballot before voting, multiply your primes together, and bring that resultant with you when voting. The tabulator at the precinct would be a relatively 'dumb' machine, which reads your filled-in bubbles, computes the product of the primes, shows that on a display for your confirmation, sends that prime number to an independent database, but also tallies the bubble counts. Several means of verification. The product of the primes gets to be a huge number, though, if there are a lot of ballot entries. Maybe breaking the ballot into Federal/state/local sections would keep the number small enough not to lose numerical significance.