  We propose two Quantum Direct Communication (QDC) protocols with user
authentication. Users can identify each other by checking the correlation of
Greenberger-Horne-Zeilinger (GHZ) states. Alice can directly send a secret
message to Bob using the remaining GHZ states after authentication. Our second
QDC protocol can be used even though there is no quantum link between Alice and
Bob. The security of the transmitted message is guaranteed by properties of
entanglement of GHZ states.

Quantum Physics