No, I mean 2FA. It could be done with Google Authenticator, being emailed a link to click, receiving a PIN via text message, or answering a security question.
People could more easily sell their votes.
That already happens, and the price is usually a couple packs of cigs or beer money. The street people who sell their votes are highly unlikely to go through the procedures to become online voters. They will be paper voters, if anything.
Experts could figure out individual voter's votes from that info.
How? The file connecting the ballet metadata to the voter would not be available to "experts" unless they show cause and obtain a court order for any inquiry.
Those are all insecure. Many security questions were made insecure by China's hack of OPM clearance data which gave them a wealth of previous addresses, cities of birth, mother's maiden name, etc. Being emailed a link is meaningless. I can create an email account in two seconds, pretend to be you, then "verify" I am you with that email. Text messages are a PITA and not secure either. I can ask a phone company to port your number to my phone. Your phone company may stop that from happening but in some cases I can call them and pretend to be you and authorize the switch.
The file connecting the ballet metadata to the voter would not be available to "experts" unless they show cause
I didn't expect that. The unique ID on each ballot can be done securely if it is long enough and random enough. Many mail-in ballots are not. They have a code that's not much longer than the number of residents in the county and even assigned randomly they can figured out through process of elimination.
More on “selling votes” via the online system I propose: The arrangement I propose would actually make it child’s play to defraud a vote buyer.
The ballot image retained by the online voter would be exactly the same as the image you would obtain by downloading a ballot image from the published list. Exactly the same. Therefore a vote buyer would have no way of knowing if the image you provide is yours or one you simply downloaded. The published list would be populated in real-time as ballots come in, so you would be able to download a matching ballot image as quickly as you could actually vote and generate your own. Buying votes from online voters would be begging to be screwed.