Frame-bitrate-change based steganography for voice-over-IP

LIU Jin(刘进)^{1, 2}, TIAN Hui(田晖)³, ZHOU Ke(周可)^{1, 2}

(1. School of Computer Science and Technology, Huazhong University of Science and Technology,
Wuhan 430074, China;
2. Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology,
Wuhan 430074, China;
3. College of Computer Science and Technology, National Huaqiao University, Xiamen 361021, China)

Abstract:Steganography based on bits-modification of speech frames is a kind of commonly used method, which targets at RTP payloads and offers covert communications over voice-over-IP (VoIP). However, direct modification on frames is often independent of the inherent speech features, which may lead to great degradation of speech quality. A novel frame-bitrate-change based steganography is proposed in this work, which discovers a novel covert channel for VoIP and introduces less distortion. This method exploits the feature of multi-rate speech codecs that the practical bitrate of speech frame is identified only by speech decoder at receiving end. Based on this characteristic, two steganography strategies called bitrate downgrading (BD) and bitrate switching (BS) are provided. The first strategy substitutes high bit-rate speech frames with lower ones to embed secret message, which introduces very low distortion in practice, and much less than other bits-modification based methods with the same embedding capacity. The second one encodes secret message bits into different types of speech frames, which is an alternative choice for supplement. The two strategies are implemented and tested on our covert communication system StegVoIP. The experiment results show that our proposed method is effective and fulfills the real-time requirement of VoIP communication.

Key words:covert communication; steganography; multi-rate speech codec; voice-over-IP (VOIP)