A generalized architecture of quantum secure direct communication for N disjointed users with authentication

In this paper, we generalize a secured direct communication process between N users with partial and full cooperation of quantum server. So, N-1 disjointed users u1, u2, .., u_N-1 can transmit a secret message of classical bits to a remote user uN by utilizing the property of dense coding and Pauli unitary transformations. The authentication process between the quantum server and the users are validated by EPR entangled pair and CNOT gate. Afterwards, the remained EPR will generate shared GHZ states which are used for directly transmitting the secret message. The partial cooperation process indicates that N-1 users can transmit a secret message directly to a remote user uN through a quantum channel. Furthermore, N-1 users and a remote user uN can communicate without an established quantum channel among them by a full cooperation process. The security analysis of authentication and communication processes against many types of attacks proved that the attacker cannot gain any information during intercepting either authentication or communication processes. Hence, the security of transmitted message among N users is ensured as the attacker introduces an error probability irrespective of the sequence of measurement.