## 1 Introduction  
Quantum key distribution (QKD) [1; 2; 3], which is one of the most widely implemented areas of quantum information, enables secure communication for legal participants over public channels. The security of QKD has been guaranteed on the theory of quantum physics against adversaries with unlimited computing power [4; 5]. Continuous variable (CV) QKD [6; 7; 8] provides secret key rates as high as half of the Pirandola-Laurenza-Ottaviani-Banchi bound with off-the-shelf devices. So far, theoretical studies and experimental implementations of CVQKD have been put forward, respectively.  
As a crucial landmark, a quantum network was proposed to extend QKD from point-to-point configuration to a multi-user and large-scale scenario. A series of quantum networks based on point-to-point QKD devices are designed and developed [9; 10; 11; 12; 13], including two principle types: quantum channel switching networks [14] and trusted-repeater-based quantum networks [15]. However, in these quantum network schemes, quantum memories are required for restoring quantum states. Moreover, there is an urgent need to address the security problems caused by the repeater nodes in the quantum networks, which may not be trusted in practical situations.  
Recent developments in the field of entanglement based multipartite QKD [16; 17; 18] have led to a renewed interest in the construction of QKD. Unlike the conventional point-to-point QKD, the CV QKD with entanglement in the middle [16] utilizes an untrusted third party to generalize Einstein-Podolsky-Rosen states, which are used for establishing the secure communication between two legitimate parties against both collective attacks and coherent attacks [18]. On this basis, the tripartite CV QKD with entanglement source in the middle is proposed for providing secure key generation of three legitimate parties against coherent attacks on both the quantum links and the untrusted source. Although some research has been carried out on the CV QKD with entanglement in the middle, the mechanism by which utilizes an untrusted entanglement source to build a quantum network with arbitrary number of users has not been established.  
Recently, an measurement-device-independent (MDI) star network based on CV systems has been proposed [19], which provides high-rate quantum conference and quantum secret sharing services of a building-sized transmission distance. However, the commercial implementation of this MDI star network is limited by the transmission distance. This paper proposes a new methodology for constructing a quantum conferencing network with an entanglement source in the middle, based on multipartite CV Greenberger-Horne-Zeilinger (GHZ) states generation and homodyne detection. Entangled modes of CV GHZ states are generated in the untrusted source in the middle, and distributed to an arbitrary number of legitimate network users. After the homodyne detections of the received modes and the postprocessing procedures, each pair of the parties share mutual information for generating multipartite quantum cryptographic conferencing key. Security analysis proves that the entanglement-based quantum conferencing scheme is able to provide secure communication against collective attacks. Moreover, by applying a Gaussian de Finetti reduction [20], the security against coherent attacks is also guaranteed [19]. Simulation results show that our scheme can establish high rate secure conferencing communication for about 100 users in a quantum network with a radius of hundreds of meters.  
This paper is organized as follows. In Section 2, we propose the multipartite quantum conferencing scheme with entanglement in the middle. In Section 3, we analyze the security of the scheme against collective attacks and provide a derivation progress for the key rate. The simulation results are shown in Section 4. Finally, we draw our conclusions in Section 5.