Continuous-variable multiparty quantum key agreement based on third party

General quantum key agreement (QKA) protocols require each participant to make the same contribution to the generation of a shared key, but this situation cannot meet the needs of government or military which require a third party only to control key agreement process. This paper proposed a continuous-variable quantum key agreement (CVQKA) protocol with multi-party Greenberg-Horne-Zeilinger (GHZ) entangled states generated by a third party. The third party prepares quantum states by sending multi-photon beam with higher emission frequency and stronger signal. Taking continuous variable as carrier, the information carried by each carrier increases, which improves the security key rate. In this paper, the third party controls but does not contribute to the final key. After receiving the states, each participant encodes his or her own key, adds a decoy state and sends it to the next participant, until receiving the quantum state that was encoded by all the participants. Then, participants can measure and calculate to get the final shared key. It shows that our protocol is easy to implement and has high efficiency. Security analysis shows that our protocol can resist collusion attacks, beam splitter attack, etc.