Improving the capacity of the Boström-Felbinger protocol

We show that the capacity of the Bostr\"om-Felbinger (ping-pong) protocol Phys. Rev. Lett. 89, 187902 (2002) can be doubled by introducing two additional unitary operations. In message mode, Alice can use one of the four unitary operations to encode two classical bits of information. Bob can decode Alice's information by using a Bell-basis measurement. Security against eavesdropping attack can be ensured by using two conjugate measurement bases in control mode. A message authentification method can protect this communication against a man-in-the-middle attack without eavesdropping.