Cryptanalysis of multiparty controlled quantum secure direct communication using Greenberger–Horne–Zeilinger state

We analyze the security of multiparty controlled quantum secure direct communication using Greenberger–Horne–Zeilinger (GHZ) state. It is shown that the receiver, using a special property of GHZ state, can illegally obtain 33.3% of the sender’s secret without any controller’s permission. The attack strategy is demonstrated in detail and an improvement of this protocol is discussed. The idea of this attack might be instructive for the cryptanalysis of quantum cryptographic protocols.