  We study the problem of simultaneously addressing both ballot stuffing and
participation privacy for pollsite voting systems. Ballot stuffing is the
attack where fake ballots (not cast by any eligible voter) are inserted into
the system. Participation privacy is about hiding which eligible voters have
actually cast their vote. So far, the combination of ballot stuffing and
participation privacy has been mostly studied for internet voting, where voters
are assumed to own trusted computing devices. Such approaches are inapplicable
to pollsite voting where voters typically vote bare handed. We present an
eligibility audit protocol to detect ballot stuffing in pollsite voting
protocols. This is done while protecting participation privacy from a remote
observer - one who does not physically observe voters during voting. Our
protocol can be instantiated as an additional layer on top of most existing
pollsite E2E-V voting protocols. To achieve our guarantees, we develop an
efficient zero-knowledge proof (ZKP), that, given a value $v$ and a set $\Phi$
of commitments, proves $v$ is committed by some commitment in $\Phi$, without
revealing which one. We call this a ZKP of reverse set membership because of
its relationship to the popular ZKPs of set membership. This ZKP may be of
independent interest.
