What is chain voting? I read The Three Ballot Voting System by Rivest. This paper-based voting system can be attacked with chain voting. But can't find any description of what "Chain voting" is mentioned in section 4.9.

Would anyone explain what it is?
After a small search; it is a vote buying scheme.


  Chain voting, a vote buying scheme in which a crook gives the voter a pre-voted ballot, the voter votes that ballot, and then after leaving the polling place, sells his blank ballot to the crook, who votes it and then gives it to the next willing participant.


This is from Douglas W. Jones web page and reference 12 of the Rivest's article points his definition.

This attack is applied in many countries (I'll not give any source). One countermeasure can be paper-based PUF. Also, some ideas from University Voting Systems Competition might be interesting to look at.