Abstract: VoIP (Voice over IP) is a kind of voice communication technology based on UDP/IP protocols, so packet loss will inevitably happen when the channel environment deteriorates. Besides that, stego VoIP data flows are real-time and have no head or tail, which pose more challenges to accurately extract the secret messages. This paper proposes a robust and self-synchronous VoIP steganography method, which solves the above two problems under the premise of maintaining good imperceptibility. In this method, low density parity check (LDPC) codes are introduced to preprocess the secret data, and the encoded data are embedded into voice packets with minimum distortion using matrix embedding. In this way, the erasure correcting capability for covert communication is enhanced. Based on the decoding characteristics and the check constraints of LDPC codes, a code-aided synchronization scheme is established, which is used in the new method to realize self-synchronous message extraction. Experimental results show that the novel method can effectively synchronize both the sender and the receiver, and its resistance to packet loss is better than existing methods.

Key words: voice over IP, adaptive steganography, packet loss recovery, steganographic synchronization