Cryptanalysis of a multiparty quantum key agreement protocol based on commutative encryption

Recently, Sun et al. (Quantum Inf Process 15(5):2101–2111, 2016) proposed an efficient multiparty quantum key agreement protocol based on commutative encryption. The aim of this protocol is to negotiate a secret shared key among multiple parties with high qubit efficiency as well as security against inside and outside attackers. The shared key is the exclusive-OR of all participants’ secret keys. This is achieved by applying the rotation operation on encrypted photons. For retrieving the final secret key, only measurement on single states is needed. Sun et al. claimed that assuming no mutual trust between participants, the scheme is secure against participant’s attack. In this paper, we show that this is not true. In particular, we demonstrate how a malicious participant in Sun et al.’s protocol can introduce “a” final fake key to target parties of his choice. We further propose an improvement to guard against this attack.