VoIP Steganography Methods, a Survey
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paper_title: Optimal matrix embedding for Voice-over-IP steganography

paper_content:

Divide-and-rule strategy is popularly adopted in steganography based on Voice-over-IP (VoIP), which often divides a cover into many small parts and performs embedding operation on each one to maintain the real-time requirement of VoIP services. No existing study, however, notices that the cover length of the parts may seriously affect the embedding performance. The goal of this paper is to fill in the gap and present a novel steganographic scheme to achieve the optimal embedding. To attain the goal, we first present an adjustable matrix encoding (AME) approach, which can adaptively generate a guide matrix to accommodate to the given cover length and provide the best performance while guaranteeing the desired embedding capacity. Further, the optimal embedding is modeled as a typical combinatorial optimization problem, of which the optimal solution is an AME or a combination of multiple AMEs. We evaluate the proposed scheme with ITU-T G. 711 (A-law) as the codec of the cover speech and compare it with previous methods. The experimental results demonstrate that the proposed scheme is really feasible in both theory and practice, and can provide consistently optimal embedding performance in any case. We present an optimal embedding scheme for Voice over IP.A novel adjustable matrix encoding (AME) is proposed to suit the limited cover.The optimal embedding using AME is modeled as a combinatorial optimization problem.An algorithm to search the optimal solution is provided.Our scheme can provide consistently optimal embedding performances in any case.
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paper_title: An Efficient VoIP Steganography Based on Random Binary Matrix

paper_content:

To reduce the distortion while maintaining relatively large embedding capacity, a steganographic scheme based on random binary matrix is presented in this paper. We first design a 3×4 covering code, which can hide 3 bits of secret messages into 4 bits of cover messages with not more than 2-bit changed. Further, we construct a random binary matrix with cover bits at diagonal lines for steganography, and conceal secret messages into each four cover bits encrypted with the other binary bits of their rows or columns using the above 3×4 covering code. The proposed scheme is evaluated with a large number of G.711 encode speech samples. The experimental results show that our scheme can provide good performance on both steganographic transparency and embedding capacity.
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paper_title: Improved adaptive partial-matching steganography for Voice over IP

paper_content:

In Voice over IP scenarios, adaptive partial-matching steganography (APMS) is an effective approach to balance steganographic transparency and bandwidth, in which the strategies involved, however, have not achieved the optimal steganographic performance yet. Therefore, in this paper, we try to improve APMS from three aspects. First, when the current partial similarity value (PSV) falls between the two given substitution thresholds, the embedding process is guided with unequal probabilities increased with the PSVs instead of identical probabilities, which enhances the embedding chances of cover parts with larger PSVs. Second, for cover parts considered unsuitable for information hiding in the original strategies, the matrix encoding strategy is employed to conceal secret messages with no more than one bit changed, which improves cover usage rate while maintaining high embedding efficiency. Third, in the APMS strategy with two flag bits, one more encrypted form of the secret message is added into the comprehensive evaluation of partial matching, which makes the embedding process more efficient. The proposed strategies are evaluated with ITU-T G.711 (A-law) as the codec of the cover speech, and compared with the original ones and some existing approaches. The experimental results demonstrate that the improved strategies can strike excellent balances between the steganographic transparency and bandwidth, and significantly outperform both the former ones and other previous approaches. Furthermore, the results of detection tests show they have outstanding performance of steganalysis resistance.
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paper_title: Hiding Information Into Voice-Over-IP Streams Using Adaptive Bitrate Modulation

paper_content:

Information hiding based on bitrate switching is a novel steganographic scheme for voice over IP, which can resist traditional steganalysis methods. However, the state-of-the-art method has not achieved its best embedding performance. Thus, in this letter, we present two adaptive bitrate modulating-based steganographic methods introducing Hamming matrix encoding (HME) strategy. The first one can conceal ${\lfloor \log _{2}(n+1)\rfloor }$ bits of secret data into ${n}$ speech frames with at most one bitrate switching by applying the HME strategy to each frame group, while the second one, compared with the state-of-the-art method, can embed ${(1+\lfloor \log _{2}(n+1)\rfloor)/2}$ bits of secret data into ${n}$ speech frames without increasing the number of bitrate switches by only employing the HME strategy for the frame groups with bitrate switching. The theoretical analysis and experimental evaluation demonstrate that the proposed methods are feasible, and outperform the existing one on both covert bandwidth and embedding efficiency while achieving the comparably good steganographic transparency.
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paper_title: Adaptive partial-matching steganography for voice over IP using triple M sequences

paper_content:

Although steganographic transparency and steganographic bandwidth are believed to be two conflicting objectives in the design of steganographic systems, it is possible and necessary to strike an optimal balance between them. This paper presents an adaptive partial-matching steganography for voice over IP (VoIP). We introduce the notion of partial similarity value (PSV) to evaluate the partial matching between covers and secret messages. By properly setting a low threshold of PSV and a high threshold of PSV, we can adaptively balance steganographic transparency and bandwidth. Moreover, we employ triple m sequences to eliminate the correlation among secret messages, guide the adaptive embedding process, and encrypt synchronization signaling patterns. In addition, we introduce an improved strategy that takes into account the similarity between not only covers and encrypted messages but also covers and original messages. We evaluate the proposed approach and its improved strategy with ITU-T G.729a as the codec of the cover speech in StegVoIP that is a prototypical covert communication system based on VoIP and compare them with some existing approaches. The experimental results demonstrate that the proposed approaches can provide a better balance between steganographic transparency and bandwidth. Furthermore, the results of delay tests show that they adequately meet the real-time requirement of VoIP.
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paper_title: SkypeLine: Robust Hidden Data Transmission for VoIP

paper_content:

Internet censorship is used in many parts of the world to prohibit free access to online information. Different techniques such as IP address or URL blocking, DNS hijacking, or deep packet inspection are used to block access to specific content on the Internet. In response, several censorship circumvention systems were proposed that attempt to bypass existing filters. Especially systems that hide the communication in different types of cover protocols attracted a lot of attention. However, recent research results suggest that this kind of covert traffic can be easily detected by censors. In this paper, we present SkypeLine, a censorship circumvention system that leverages Direct-Sequence Spread Spectrum (DSSS) based steganography to hide information in Voice-over-IP (VoIP) communication. SkypeLine introduces two novel modulation techniques that hide data by modulating information bits on the voice carrier signal using pseudo-random, orthogonal noise sequences and repeating the spreading operation several times. Our design goals focus on undetectability in presence of a strong adversary and improved data rates. As a result, the hiding is inconspicuous, does not alter the statistical characteristics of the carrier signal, and is robust against alterations of the transmitted packets. We demonstrate the performance of SkypeLine based on two simulation studies that cover the theoretical performance and robustness. Our measurements demonstrate that the data rates achieved with our techniques substantially exceed existing DSSS approaches. Furthermore, we prove the real-world applicability of the presented system with an exemplary prototype for Skype.
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paper_title: Real-Time Covert VoIP Communications over Smart Grids by Using AES-Based Audio Steganography

paper_content:

Smart grid has been spread worldwide, but its data security and privacy have not got enough attention. In this paper, we propose a VoIP steganography method based on AES to achieve a real-time covert communication over smart grids. The covert communication system is implemented by embedding secret message encrypted with symmetric cryptography AES into an audio signal encoded by PCM codec. In order to meet the requirement of real-time, we encrypt the secret message and embed it with different algorithms into audio packets before sending them. So observers cannot detect it by using a simple statistical analysis. At the receiving end, we employ the corresponding method to retrieve the secret message, and then decrypt it to get the original message. For different embedding methods, each packet has a different capacity for embedding the secret message.
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paper_title: Steganography in low bit-rate speech streams based on quantization index modulation controlled by keys

paper_content:

Since low bit-rate speech codecs used for voice over internet protocol (VoIP), such as iLBC (internet low bit-rate codec), G.723.1 and G.729A, have less redundancy due to high compression, it is more challenging to embed information in low bit-rate speech streams of VoIP. In this study, a new method is proposed for steganography in low bit-rate speech streams of VoIP. The core idea of this method is setting up a graph model for the codebook space of the quantizer. Based on the graph model, the method realises a quantization index modulation (QIM)-controlled algorithm for partitioning the codebook space. It can be proved that this method can minimize signal distortion while steganography taking place. Taking into account codeword partition balance and partition diversity, the proposed steganographic algorithm was based on QIM controlled by secret keys, i.e., mapping the ways of codebook division into secret keys, thereby significantly improving the undetectability and robustness of VoIP steganography. Performance measurements and steganalysis experiments showed that the proposed QIM-controlled steganographic algorithm was more secure and robust than the QIM algorithm, the conventional RANDOM algorithm and the original codebook algorithm.
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paper_title: Improving security of quantization-index-modulation steganography in low bit-rate speech streams

paper_content:

In this study, we mainly concentrate on quantization-index-modulation (QIM) steganography in low bit-rate speech streams, and contribute to improve its security. Exploiting the characteristics of codebook division diversity in the complementary neighbor vertices algorithm, we first design a key-based codebook division strategy, which follows Kerckhoff's principle and provides a better security than the previous QIM approach. Further, to resist the state-of-the-art steganalysis, following a general belief that fewer and smaller cover changes are less detectable and more secure, we present an improved QIM steganography, which introduces random position selection to adjust the embedding rate dynamically, and employs matrix encoding strategy to enhance the embedding efficiency. The proposed approach is evaluated with ITU-T G.723.1 as the codec of cover speech and compared with the previous work. The experimental results demonstrate that the proposed approach outperforms the traditional QIM approach on both steganographic transparency and steganalysis resistance. Moreover, it is worth pointing out that our approach can effectively work in conjunction with not only G.723.1 codec but also all other parametric speech coders, and be successfully applied into Voice-over-Internet-Protocol systems.
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paper_title: Neighbor-index-division steganography based on QIM method for G.723.1 speech streams

paper_content:

Quantization index modulation (QIM) is a commonly used steganography method exploiting redundancy in codebook quantization procedure of signal encoding. The feasibility of steganography based on QIM depends on codebook division algorithm which mainly determines embedding capacity, latency and complexity of embedding and extracting algorithm. A novel neighbor-index-division (NID) codebook division method with a simple and more security implementation for G.723.1 speech streams is proposed. Differing from existing complementary neighbor vertices (CNV) algorithm, NID divides neighbor-indexed codewords into separated sub-codebooks (partitions) combined with a suitable stego coding strategy. Moreover, NID method makes multiple division (k sub-codebooks, k ≥ 3) for one single codebook possible, thus increases embedding capacity. To take full advantage of the partitions especially for odd-numbered division, multi-ary coding strategy is introduced in embedding and extracting procedures. Experimental results demonstrate that proposed method has almost the same distortion compared with CNV in binary division condition but a much easier implementation and introduces flexible multiple division methods which is more suitable for practical covert communication applications. Further, by applying a commonly used COV(1, 3, 2) stego coding strategy, NID is able to resist the state-of-art steganalysis towards QIM with a still practical embedding rate.
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paper_title: Pitch-based steganography for Speex voice codec

paper_content:

This paper presents an improved version of a steganographic algorithm for IP telephony called HideF0. It is based on approximating the F0 parameter, which is responsible for conveying information about the pitch of the speech signal. The bits saved due to simplification of the pitch contour are used for the hidden transmission. In our experiments, the proposed method was applied to the narrowband Speex codec working in five different modes, with bitrates between 5,950i?źbps and 24,600i?źbps. We showed that HideF0 was able to create hidden channels with steganographic bandwidths of around 200i?źbps at the expense of a steganographic cost of between 0.5 and 0.7 MOS, depending on the Speex mode. Because of placing the approximation flag in the voice packet header, the improved version of the proposed algorithm yielded a significantly lower decrease in speech quality, when compared with the original version of HideF0. In addition, for low bitrates of the hidden channel i.e., below ca. 50i?źbps it was able to operate without introducing any steganographic cost. Copyright © 2016 John Wiley & Sons, Ltd.
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paper_title: Novel Method of Hiding Information in IP Telephony Using Pitch Approximation

paper_content:

In this paper a novel steganographic method, called Hide F0, dedicated to IP telephony is proposed. It is based on the approximation of the parameter that describes the F0 frequency (the pitch) of the speaker's voice. We show that thanks to approximating some fragments of the "fine pitch" parameter in the Speex codec we can create efficient hidden transmission channels. We determined that for Speex working in mode 5 the Hide F0 method can provide a hidden channel with a capacity of ca. 220 bps at the optimal operating point. We also demonstrated that the proposed method offers a significantly more advantageous trade-off between the steganographic bandwidth and steganographic cost than the classic least significant bit (LSB) approach.
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paper_title: Covert voice over internet protocol communications based on spatial model

paper_content:

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 steganography in media streams. Experimental results on VoIP steganography show that the proposed 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.
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paper_title: Covert Voice over Internet Protocol Communications with Packet Loss Based on Fractal Interpolation

paper_content:

The last few years have witnessed an explosive growth in the research of information hiding in multimedia objects, but few studies have taken into account packet loss in multimedia networks. As one of the most popular real-time services in the Internet, Voice over Internet Protocol (VoIP) contributes to a large part of network traffic for its advantages of real time, high flow, and low cost. So packet loss is inevitable in multimedia networks and affects the performance of VoIP communications. In this study, a fractal-based VoIP steganographic approach was proposed to realize covert VoIP communications in the presence of packet loss. In the proposed scheme, secret data to be hidden were divided into blocks after being encrypted with the block cipher, and each block of the secret data was then embedded into VoIP streaming packets. The VoIP packets went through a packet-loss system based on Gilbert model which simulates a real network situation. And a prediction model based on fractal interpolation was built to decide whether a VoIP packet was suitable for data hiding. The experimental results indicated that the speech quality degradation increased with the escalating packet-loss level. The average variance of speech quality metrics (PESQ score) between the “no-embedding” speech samples and the “with-embedding” stego-speech samples was about 0.717, and the variances narrowed with the increasing packet-loss level. Both the average PESQ scores and the SNR values of stego-speech samples and the data-retrieving rates had almost the same varying trends when the packet-loss level increased, indicating that the success rate of the fractal prediction model played an important role in the performance of covert VoIP communications.
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paper_title: A New Data-Hiding Approach for IP Telephony Applications with Silence Suppression

paper_content:

Even if information hiding can be used for licit purposes, it is increasingly exploited by malware to exfiltrate data or to coordinate attacks in a stealthy manner. Therefore, investigating new methods for creating covert channels is fundamental to completely assess the security of the Internet. Since the popularity of the carrier plays a major role, this paper proposes to hide data within VoIP traffic. Specifically, we exploit Voice Activity Detection (VAD), which suspends the transmission during speech pauses to reduce bandwidth requirements. To create the covert channel, our method transforms a VAD-activated VoIP stream into a non-VAD one. Then, hidden information is injected into fake RTP packets generated during silence intervals. Results indicate that steganographically modified VAD-activated VoIP streams offer a good trade-off between stealthiness and steganographic bandwidth.
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paper_title: Improving security of quantization-index-modulation steganography in low bit-rate speech streams

paper_content:

In this study, we mainly concentrate on quantization-index-modulation (QIM) steganography in low bit-rate speech streams, and contribute to improve its security. Exploiting the characteristics of codebook division diversity in the complementary neighbor vertices algorithm, we first design a key-based codebook division strategy, which follows Kerckhoff's principle and provides a better security than the previous QIM approach. Further, to resist the state-of-the-art steganalysis, following a general belief that fewer and smaller cover changes are less detectable and more secure, we present an improved QIM steganography, which introduces random position selection to adjust the embedding rate dynamically, and employs matrix encoding strategy to enhance the embedding efficiency. The proposed approach is evaluated with ITU-T G.723.1 as the codec of cover speech and compared with the previous work. The experimental results demonstrate that the proposed approach outperforms the traditional QIM approach on both steganographic transparency and steganalysis resistance. Moreover, it is worth pointing out that our approach can effectively work in conjunction with not only G.723.1 codec but also all other parametric speech coders, and be successfully applied into Voice-over-Internet-Protocol systems.
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Title: VoIP Steganography Methods, a Survey

Section 1: Introduction
Description 1: This section introduces the concept of steganography, particularly focusing on VoIP steganography, the dynamic nature of VoIP communication, its advantages for data hiding, and the challenges posed by data embedding in VoIP packets.

Section 2: Overview of the existing methods
Description 2: This section provides a brief description of the mechanism of VoIP, data hiding methods, and analysis of various data hiding methods based on a survey by M a z u r c z y k.

Section 3: Mechanism of VoIP
Description 3: This section explains the fundamentals of VoIP technology, detailing its packet-switching methodology and the protocols involved in its signaling and conversion phases.

Section 4: Data hiding methods in VoIP
Description 4: This section describes various domains of digital audio steganography, such as temporal, transform, and coded domains, and the different methods and algorithms used for data hiding in VoIP packets.

Section 5: Least Significant Bit (LSB) modification methods
Description 5: This section focuses on the LSB modification method, its implementation in VoIP packets, advantages like high embedding capacity, and drawbacks like vulnerability to steganalysis.

Section 6: Spread spectrum techniques
Description 6: This section discusses the use of spread spectrum techniques like Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) for data hiding in VoIP communications.

Section 7: Pulse code modulation technique
Description 7: This section explores the technique of embedding secret data in VoIP packets through pulse code modulation, including experimental methods and their results related to speech quality and embedding capacity.

Section 8: Quantization Index Modulation (QIM)
Description 8: This section details the QIM method, its process, and the development of codebooks for embedding secret messages, focusing on improving undetectability and robustness.

Section 9: Pitch modification method
Description 9: This section covers methods that modify pitch parameters (e.g., F0 parameter) in VoIP packets to hide data, including examples of applications and their outcomes.

Section 10: Packet loss methods
Description 10: This section describes methods that exploit packet loss in VoIP communication to hide data, such as the LACK technique and other advanced algorithms to handle packet loss efficiently.

Section 11: Silence suppression methods
Description 11: This section elaborates on methods that utilize silent intervals in VoIP communication for data hiding, including modifications to packet header fields and the challenges faced.

Section 12: Codec specific methods
Description 12: This section investigates codec-specific methods where certain codecs are used for embedding secret messages during the compression process or by modifying codec parameters.

Section 13: Evaluation metrics
Description 13: This section outlines various metrics used to evaluate the quality of VoIP steganography methods, such as Embedding Rate, Bit Change Rate, Embedding Efficiency, Codeword Change Rate, PESQ, and MOS-LQO.

Section 14: Conclusion
Description 14: This section concludes the survey by summarizing the observations made from various VoIP steganography methods and discussing their strengths and weaknesses, providing insights for future research and enhancements.