# Steganalysis of Quantization Index Modulation Steganography in G.723.1 Codec
## 1 Introduction  
Steganalysis and steganography are a pair of game relationships. Just like cryptanalysis and cryptography, they are opponents, but they are closely related and complement each other. Steganography is an important branch of information hiding technology. It embeds the secret information into the carrier (such as text, image, voice, video, etc.) according to a certain method, and transmits the confidential information to the receiver without being perceived by the third party, and completes the secret information communication. Steganography not only guarantees that secret information is transmitted securely, but also ensures the integrity of the carrier. On the contrary, steganalysis detects whether the carrier contains secret information, which can block the transmission of illegal information and ensure communication security.  
With the rapid development and widespread application of steganography, it has been used by illegal criminals on the Internet, infringing individual privacy and endangering social security. For example, while voice over internet protocol (VoIP) brings convenience to people's lives, it is also used by illegal steganography users for cybercrime (to deliver illegal information via VoIP) [1]. Under such a circumstance, this paper conducts steganalysis of the secret concealment information in the coding standard G.723.1 that commonly used in VoIP, and presents a steganalysis method based on quantization index modulation (QIM).  
In the method of implementing steganography using an encoder, it is particularly preferred to adopt QIM. The reasons are as follows [2; 3].  
QIM steganography combines its own quantization with vector quantization in the process of speech coding. Encoding and concealing are integrated and completed at the same time, which forms the perfect hiding of secret information and greatly reduces the coding delay. In particular, the complementary neighbor vertex-quantization index modulation (CNV-QIM) algorithm was proposed by [PERSON] from Tsinghua University, and the information steganography performance is higher than the QI algorithm.  
Grouping the quantized vector codebooks and then performing QIM steganography can minimize distortion and ensure hidden voice quality. In view of the above reasons, the speech effect based on QIM steganography is very ideal, and it is very difficult to analyse its steganography.  
Existing research work on steganalysis of speech encoders has achieved a lots results. In particular, the teams of Professor [PERSON] and Professor [PERSON] of Tsinghua University have conducted a lot of research on speech coding steganography analysis for VoIP. They obtained some progress in G.729 [2] and G.723 [3] steganalysis. Since QIM steganography inevitably causes changes in the statistical characteristics of speech coding, steganalysis can be performed using this change. Therefore, the research in this paper is a steganalysis of the changes of some statistical features caused by QIM steganography in G.723.1 speech coding.  
The rest of the paper is organized as follows: Previous related work is reviewed in Section 2. The change of the index distribution characteristics before and after QIM steganography are discussed in Section 3. In Section 4, we study the quantization method of index distribution characteristics and use PCA to reduce the dimensionality and design the SVM classifier. Experimental results and discussion are shown in Section 5. Finally, concluding remarks are given in Section 6, the following research work is also introduced in this section.