External attack of a quantum secret sharing scheme

This paper analyzes the security of a quantum secret sharing protocol between multiparty and multiparty, which was proposed by Yan and Gao. We show that this protocol is not secure because an external attacker can obtain the secret message shared by the two groups, making use of entanglement swapping. There are two reasons leading to this result. Firstly, the messages, which are transmitted on the quantum and classical channels, are not protected by message authentication. Secondly, the last participant in group 1 knows more information about the shared secret by the two groups. This result shows that there are some hidden security loopholes in this kind of quantum secret sharing protocols.