  A continuous variable measurement device independent multi-party quantum
communication protocol is investigated in this paper. Utilizing distributed
continuous variable Greenberger-Horne-Zeilinger state, this protocol can
implement both quantum cryptographic conference and quantum secret sharing. We
analyze the security of the protocol against both entangling cloner attack and
coherent attack. Entangling cloner attack is a practical individual attack, and
coherent attack is the optimal attack Eve can implement. Simulation results
show that coherent attack can greatly reduce the secret key rate. Different
kinds of entangled attacks are compared and we finally discuss the optimal
coherent attacks.
