This paper presents a new spatial steganography model for covert communications over Voice over Internet Protocol (VoIP), providing a solution to the issue of increasing the capacity of covert VoIP channels without compromising the imperceptibility of the channels. Drawing from Orthogonal Modulation Theory in communications, the model introduced two concepts, orthogonal data hiding features and data hiding vectors, to covert VoIP communications. By taking into account the variation characteristics of VoIP audio streams in the time domain, a hiding vector negotiation mechanism was suggested to achieve dynamic self-adaptive ste-ganography in media streams. Experimental results on VoIP steganography show that the pro-posed steganographic method effectively depicted the spatial and temporal characteristics of VoIP audio streams, and enhanced robustness against detection of steganalysis tools, thereby improving the security of covert VoIP communications
