The wireless networks lack a physical boundary due to the broadcasting nature of wireless transmissions. The security has become a critical concern in the physical layer of wireless networks. In this work, we present a cross-layer security scheme for STBC system. By introducing a distort signal set the sender randomly flip-flops between the distort signal set and the orthogonal code set to confuse the attacker. The physical-layer security is enhanced as a result. In the proposed scheme the physical-layer may rely on upper-layer encryption techniques for security, which results in a cross-layer security scheme.