DMV has voter registration rolls
It's agreed that everyone could get a mail-in voter form in the mail.
Just mail every registered voter a private key
at each DMV have a public key associated with each registered voter
the private key has to match the public key or the vote won't submit
everyone can log on and vote from home.
if more than "x" votes per home, red-flag is raised.
This is about as secure as you're going to get. It's how Google verifies user authenticity currently. Each IP gets logged to the voter for audit purposes. Even before the voting date, people could request 2-way verification be sent to their cell phones.