  Two quantum key agreement protocols using Bell states and Bell measurement
were recently proposed by Shukla et al.(Quantum Inf. Process. 13(11),
2391-2405, 2014). However, Zhu et al. pointed out that there are some security
flaws and proposed an improved version (Quantum Inf. Process. 14(11),
4245-4254, 2015). In this study, we will show Zhu et al.'s improvement still
exists some security problems, and its efficiency is not high enough. For
solving these problems, we utilize four Pauli operations {I, Z, X, Y } to
encode two bits instead of the original two operations {I,X} to encode one bit,
and then propose an efficient and secure arbitrary N-party quantum key
agreement protocol. In the protocol, the channel checking with decoy single
photons is introduced to avoid the eavesdropper's flip attack, and a
post-measurement mechanism is used to prevent against the collusion attack. The
security analysis shows the present protocol can guarantee the correctness,
security, privacy and fairness of quantum key agreement.
