Optical scan voting

This is the basic model of optical scan voting as currently practiced by the US. There are some proposed improvements the current system (such as Scantegrity) that would still use optical scanners for tallying, but they are discussed in their relevant articles. This basic model of optical scan voting machines involves voters casting their votes on paper, with their ballot later being scanned by an optical scan machine. The optical scan machine can then be audited by the paper record.

Infrastructure attack protection
Little infrastructure is needed for people to vote using an optical scan system. Even if the power's out or if equipment fails, voters can vote on paper and have the optical scanner record it later.

Outsider hacking protection
It would be very hard for an adversary to infiltrate an elections building as to modify its optical scan machine(s). Yet that doesn't mean it's impossible. In addition to these chances being very slim, there is a paper record to serve as a backup and as an auditing tool of the optical scanners.

Malware and virus protection
It would seem to be the case that VBM would be invulnerable to viruses. Yet this isn't the case - just as DREs can be compromised, so can optical scan systems. Election staffers may notice this even if the paper record is in front of them. Adding to the problem is the fact that 1) paper is bad as a recording medium and 2) it can be unclear when threats are sufficiently bad to completely scrap the optical scan system. Having a paper record is a good thing, but it's useless unless sufficient attention is paid to the optical scan machines in the first place.

Man in the middle attack protection
This would be hard - the optical scan machines would be difficult to fiddle with. But even then is the issue of doing so without getting caught which is a problem as the adversary could be found out via either audits or by security guards.

Insider attack protection
Optical scan machines are vulnerable to insider attacks, as the firmware can easily be tampered with. This is mitigated by 1) the paper ballot record and 2) checks on individual election staffers. But even with such an audit trail it can be unclear when threats are sufficiently bad to completely scrap the optical scan system. Having a paper record is a good thing, but it's useless unless sufficient attention is paid to the optical scan machines in the first place.

Coercion resistance
Optical scan voting has a high level of protection against vote selling because it is essentially receipt free. It's impossible to create a verifiable record that someone has voted in a certain way given that voters fill out their ballots behind a curtain. Even if voters could sneak a camera in and take pictures or a video of their ballot, they could always just cross out that vote (while still behind the curtain) and bubble in another candidate. A technology that solves this problem (i.e. a hidden shirt camera that is always recording) would still make it hard to sell votes, because the "receipts" generated this way would be hard to verify.

Ensuring one person, one vote
Paper ballots solve this "because each voter is given only a single paper ballot when they sign in." Existing optical scan systems have many security measures like serial numbers and watermarks as to prevent ballot stuffing.

Counting and tallying accuracy
A report from the Caltech/MIT VTP shows that optical scan ballots deliver the lowest rate of invalid votes of any technology form 1988-2001. Voters are asked for confirmation if they undervote. And this technology is improving: during the Minnesota Senate race in 2009 gross accuracy was 99.91%.

Voter anonymity
In person optical scan voting systems are anonymous because the ballot contains no information about the identity of the voter. The voter signs in at a polling place, but there is no way for those signatures to be mapped to the content of individual votes.

Voter verifiability
Paper ballots are voter verified in a sense because they are marked directly by the voter. But there's no way for a voter to verify that their vote was counted correctly.

Immediate results protection
Having no intermediate results before the voting period ends is not very significant for a non-absentee voting system. Even if there aren't any practical restrictions to prevent obtaining intermediate results, straw polls in the status quo achieve the same end of producing intermediate results on election day. These are usually accurate,though not so in the 2004 election.

Ease of performing a recount
It should be possible to perform a recount, however it is still very cumbersome to do so. There's human error involved and it may be unclear when it's necessary for a recount. These aren't major problems however.

Usability
While there are costs associated with voting this way, in person paper voting with an optical scanner is easy enough that most anyone can use it.