An effective covert communication system can be achieved based on steganography in stream media such as VoIP (Voice over IP), and the LSB-based (Least-Significant-Bit) methods are the most popular strategies. However, LSB embedding in the flat regions of the speech could inevitably bring about the perceptible distortion leading to the degradation of speech quality. Besides, LSB methods are vulnerable for manifold corresponding steganalysis like RS detection. This paper presents an adaptive steganography scheme that selects lower embedding bit rate in the flat segments, while the sharp segments can camouflage more secret message. Further, synchronous overflow judgment and the tradeoff between hiding requirement and speech quality are also taken into account to construct a practical covert communication system on G.711 VoIP. Extensive experiments showed that the proposed adaptive scheme outperformed the LSB and could evade RS steganalysis. This proposed scheme is efficient, simple and can be used in real-time VoIP network with high hiding capacity.
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