在即時溝通系統中，即時音訊隱藏指的是將秘密音訊藏入掩護音訊。藉由將秘密音訊藏入掩護音訊中，我們可以得到一個聽起來有意義並且與掩護音訊相同的隱匿音訊。因此，即使攻擊者在網際網路上擷取到音訊封包，他仍然無法察覺到內容含有秘密音訊。本篇論文提出了一種即時音訊隱藏的方法，應用在Voice over Internet Protocol (VoIP)的即時通訊系統上。
G.711是在VoIP裝置中最被廣為支援編碼技術；我們針對G.711提出了一種新的即時音訊隱藏設計。於Scheme 1實驗顯示此演算法的執行時間為0.257毫秒，這個結果證明此方法在即時VoIP應用上是適用的。我們進一步導入Advanced Encryption Standard（AES）加密演算法以提高系統的安全性，在Scheme 2的實驗中需要0.28毫秒的執行時間，也同樣顯示此方法適用於即時VoIP應用。

Speech hiding is a powerful information protection mechanism in real-time communication systems. By hiding one secret speech into the cover speech, we can get a stego speech, which sounds meaningful and indistinguishable from the original cover speech. Therefore, even if attackers intercept the audio packets on Internet, they would not notice that there is another speech hidden inside those packets. In this thesis, we propose a scheme for speech hiding in a real-time communication system such as voice over Internet Protocol (VoIP).
We propose a novel design of real-time speech hiding for G.711 codec, which is widely supported by almost every VoIP device. Two schemes were proposed in this thesis. Experimental results in Scheme 1 show that the running time for the proposed algorithm takes only 0.257ms, which is suitable for real-time VoIP applications. By adding AES encryption to enhance the security, we obtain Scheme 2 whose running time is 0.28ms, which also shows that this scheme is suitable for real-time applications.

中文摘要 2
Abstract 3
Table of Contents 4
Figure Index 6
Table Index 7
1. Introduction 8
1.1 Motivation 8
1.2 Related work 9
1.3 Solution 10
2. Scheme 1 11
2.1 Speech Hiding Space 12
2.2 Compressing Secret Speech 13
2.3 Hiding Secret Speech into Cover Speech 15
2.4 Extracting Secret Speech from Stego Speech 15
2.5 Linphone implementation 16
2.5.1 Call Setup in mediastreamer2 17
2.5.2 Modifying Codes 19
2.6. G.711 μ-law and A-law 20
3. Quality Analysis of Scheme 1 21
3.1 Scheme 1 (r=3) and Scheme 1 (r=1) 21
3.2 Signal/Noise Ratio 21
3.3 Processing Time 23
3.4 Summary 24
4. Scheme 2 25
4.1 AES Encryption/Decryption 26
4.2 Speech Hiding Space 27
4.3 Compressing Secret Speech 29
4.4 Encrypting the Compressed Secret Speech 29
4.5 Decrypting the Encrypted Secret Speech 29
4.6 Linphone implementation 30
4.7 G.711 μ-law and A-law 31
5. Quality Analysis of Scheme 2 32
5.1 Signal/Noise Ratio 32
5.2 Processing Time 32
5.3 Summary 34
6. Conclusion and Future Work 35
Reference 36
Appendix 37
Appendix 1. Codes of Scheme 1 37
Appendix 2. Codes of Scheme 2 44

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[http://www.cnss.gov/Assets/pdf/cnssp_15_fs.pdf]
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5.EasyBMP [http://easybmp.sourceforge.net/steganography.html]
6.Dorian A. Flowers, “Investigating Steganography”, ACM-SE 42, April 2004
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8.ITU-T Recommendation G.711. Pulse Code Modulation for voice frequencies, Nov. 1988.
9.Daniel Collins, Carrier Grade Voice over IP, 2nd Ed., McGraw-Hill, September 2002.
10.Speex [http://www.speex.org/]
11.J.P. Campbell, Jr., T.E. Tremain and V.C. Welch, The Federal Standard 1016 4800 bps CELP voice coder, Digital Signal Processing 1:145–154 (1991).
12.Speex Reference Manual 1.2-beta2
[http://www.speex.org/docs/api/speex-api-reference.pdf]
13.Linphone [http://www.linphone.org/]
14.FIP PUB 197 : the official AES standard
[http://www.csrc.nist.gov/publications/fips/fips197/fips-197.pdf]
15.Implements of AES (Rijndael) in C/C++ and Assembler, Brian Gladman
[http:// fp.gladman.plus.com]