  Users often wish to participate in online groups anonymously, but misbehaving
users may abuse this anonymity to spam or disrupt the group. Messaging
protocols such as Mix-nets and DC-nets leave online groups vulnerable to
denial-of-service and Sybil attacks, while accountable voting protocols are
unusable or inefficient for general anonymous messaging.
  We present the first general messaging protocol that offers provable
anonymity with accountability for moderate-size groups, and efficiently handles
unbalanced loads where few members have much data to transmit in a given round.
The N group members first cooperatively shuffle an NxN matrix of pseudorandom
seeds, then use these seeds in N "pre-planned" DC-nets protocol runs. Each
DC-nets run transmits the variable-length bulk data comprising one member's
message, using the minimum number of bits required for anonymity under our
attack model. The protocol preserves message integrity and one-to-one
correspondence between members and messages, makes denial-of-service attacks by
members traceable to the culprit, and efficiently handles large and unbalanced
message loads. A working prototype demonstrates the protocol's practicality for
anonymous messaging in groups of 40+ member nodes.
