Cao, H. & Ma, W. Multi-party quantum key agreement based on quantum search algorithm. Sci. reports 7, 45046 (2017). Many key exchange systems allow one party to generate the key and send that key simply to the other party – the other party has no influence on the key. Using a key-agreement protocol avoids some key distribution issues related to these systems. Below we analyze the collusive attack. The worst part is that only one participant is honest and everyone is dishonest. Take for example three participants P1, P2 and P3, where P1 and P3 are dishonest. They want to pre-order the last key of P2.

The detailed attack strategies are as follows. P1 prepares Bell states and sends the photon sequence | BS21?2 to P2. After P2 completes its operations on the photonic sequence, | BS21?2 and sends the sequence | BS21?3 for P3, P1, and P3 only measure Bell states at step (5) in which each participant publishes their additional control sequences. After P2 has received the RH21 command sequence from P2, P1 and P3 can deduce the secret key K2 from P2. However, the only method for P1 and P3 to determine the final key of P2 is to announce false command sequences. Based on the analysis of the attack strategy of the first participant, we can conclude the probability that he will pass the security check and that the last key of P2 is close to 0. Therefore, dishonest participants cannot determine the final key. In summary, our proposed protocol can withstand the attacks of the participants. Wang, S., Xu, G.B., Liang, X. Q. &Wu, Y.

L. Multiparty Quantum Key Agreement with 4 qubits-symmetric w-state. Int. J. Theor. 57, 3716–3726 (2018). In this article, we find that some existing MQKA protocols in travel mode are generally sensitive to internal dishonest participants. We also find the problem of information leaks in the MQKA cad-ma protocol. Next, we take the MQKA Cao-Ma protocols as examples to illustrate these attacks in detail. To resist attacks, we offer a robust MQKA protocol in travel mode based on non-orthogonal bell states. Analyses show that our protocol can withstand both external attacks and participants and achieve greater efficiency.

Finally, we design an optical platform for each participant and show that our proposed protocol can be achieved with feasible technologies.. . . .