  The DC-nets approach to anonymity has long held attraction for its strength
against traffic analysis, but practical implementations remain vulnerable to
internal disruption or "jamming" attacks requiring time-consuming tracing
procedures to address. We present Verdict, the first practical anonymous group
communication system built using proactively verifiable DC-nets: participants
use public key cryptography to construct DC-net ciphertexts, and knowledge
proofs to detect and detect and exclude misbehavior before disruption. We
compare three alternative constructions for verifiable DC-nets, one using
bilinear maps and two based on simpler ElGamal encryption. While verifiable
DC-nets incurs higher computation overheads due to the public-key cryptography
involved, our experiments suggest Verdict is practical for anonymous group
messaging or microblogging applications, supporting groups of 100 clients at 1
second per round or 1000 clients at 10 seconds per round. Furthermore, we show
how existing symmetric-key DC-nets can "fall back" to a verifiable DC-net to
quickly identify mis- behavior improving previous detections schemes by two
orders of magnitude than previous approaches.
