Cryptanalysis and improvement of the controlled secure direct communication

This paper points out that, due to a flaw in the sender's encoding, the receiver in Gao et al.'s controlled quantum secret direct communication (CQSDC) protocol Chin. Phys. 14 (2005), No. 5, p. 893 can reveal the whole secret message without the permission from the controller. An improvement is proposed to avoid this flaw.