  Quantum Key Agreement (QKA) signifies that two or more participants together
generate a key and QKA has to satisfy the following conditions: 1 Every
participant can change the key and the key is not decided by any participant
individually. 2 Only participants can know the key; nonparticipants cannot get
the key through illegal means. Because of the condition 1 of participating
together, it makes transport inefficient in the current mainstream protocols.
They use unicast to exchange messages one by one, so it will considerably limit
transmission efficiency and increase cost time spent. This study proposes a
protocol based on Multiparty Quantum Secret Direct Communication (MQSDC) with
multicast. In addition to satisfying the above conditions, it uses multicast to
not only achieve the effect and purpose of QKA, but also to defend against
internal and external attacks at the same time. In regard to resource
consumption, this study involves linear growth and is more efficient than other
mainstream protocols which employ exponential growth.
