  A large payload bidirectional quantum secure direct communication (BQSDC)
protocol without information leakage is proposed, which is based on
entanglement swapping between any two GHZ states. Two remote authorized
parties, Alice and Bob, can safely exchange their individual secret messages
without worrying about the information leakage problem. Our protocol uses a
shared secret GHZ state to overcome the information leakage problem. The shared
secret GHZ state plays two roles in the bidirectional communication process: on
one hand, it lets Bob know the prepared initial state; on the other hand, it is
used for encoding Bob's secret messages. Moreover, our protocol can transmit
six bits of secret messages per round communication. Compared with those
previous BQSDC protocols, the advantage of our protocol lies in having the
following two characters simultaneously: on one hand, it overcomes the
information leakage problem; one the other hand, its capacity is as high as six
bits per round communication.
