  In this paper, we first point out that some recently proposed quantum direct
communication (QDC) protocols with authentication are vulnerable under some
specific attacks, and the secrete message will leak out to the authenticator
who is introduced to authenticate users participating in the communication. We
then propose a new protocol that is capable of achieving secure QDC with
authentication as long as the authenticator would do the authentication job
faithfully. Our quantum protocol introduces a mutual authentication procedure,
uses the quantum Bell states, and applies unitary transformations in the
authentication process. Then it exploits and utilizes the entanglement swapping
and local unitary operations in the communication processes. Thus, after the
authentication process, the client users are left alone to communicate with
each other, and the authenticator has no access to the secrete message. In
addition, our protocol does not require a direct quantum link between any two
users, who want to communicate with each other. This may also be an appealing
advantage in the implementation of a practical quantum communication network.
