Abstract
Secure quantum key agreement schemes should satisfy four security features, namely correctness, security, privacy, and fairness. Recently, Cai et al. (2018) used five-qubit Brown states to propose an efficient and flexible multi-party quantum key agreement scheme. We review and analyze the security of their protocol and we prove its inability to secure the private information of participants against a proposed strategy of collusion attacks. Also, we suggest a simple solution to close the loophole in Cai et al.’s protocol.