Quantum secure direct dialogue using Einstein-Podolsky-Rosen pairs

A two-step quantum secure direct dialogue protocol using Einstein-Podolsky-Rosen (EPR) pair block is proposed. In the protocol, the dialogue messages are encoded on series of qubits and sent through a quantum channel directly. The security of the protocol is assured by its connection to the two-step quantum secure direct communication protocol, which has been proved secure. This protocol has several advantages. It is a direct communication protocol that does not require a separate classical communication for the ciphertext. It has high capacity as two bits of secret messages can be transmitted by an EPR pair. As a dialogue protocol, the two parties can speak to each other either simultaneously or sequentially.