  Based on an EPR pair of qubits and allowing asymptotically secure key
distribution, a secure communication protocol is presented. Bob sends either of
the EPR pair qubits to Alice. Alice receives the travel qubit. Then she can
encode classical information by local unitary operations on this travel qubit.
Alice send the qubit back to Bob. Bob can get Alice's information by
measurement on the two photons in Bell operator basis. If Eve in line, she has
no access to Bob's home qubit. All her operations are restricted to the travel
qubit. In order to find out which opeartion Alice performs, Eve's operation
must include measurements. The EPR pair qubits are destroyed. Bob's measurement
on the two photons in Bell operator basis can help him to judge whether Eve
exist in line or not. In this protocal, a public channel is not necessary.
