  As the most promising application in VANETs, the vehicular announcement
allows vehicles to send announcement messages about road conditions to other
vehicles far away. The security requirements of reliability and privacy in the
vehicular announcement are not easily achieved simultaneously due to the
notorious sybil attack. In this paper, we present a novel privacy-preserving
vehicular announcements aggregation scheme. The proposed scheme provides
threshold authentication and flexible anonymity using message aggregation and
interactive threshold ring signature which allows nondeterministic different
signers to generate signature commonly in the environment not fully trusted.
Different from existing works in an attack-then-trace mode, our scheme defends
against the sybil attack beforehand. To our best knowledge, it is the first
privacy-preserving scheme preventing the malicious users from launching the
sybil attack in advance. Through extensive evaluation, we show the availability
and the efficiency of the proposed scheme.
