Networking, Sensing and Control, 2008. ICNSC 2008. IEEE International Conference H.F. Qi, X.H. Yang, R. Jiang, B. Liang and S.J. Zhou

1. NOVEL END-TO-END VOICE ENCRYPTION
METHOD IN GSM SYSTEM
Networking, Sensing and Control, 2008. ICNSC 2008.
IEEE International Conference
H.F. Qi, X.H. Yang, R. Jiang, B. Liang and S.J. Zhou
Advisor : Dr. Jenq-Shiou Leu
Student：Bing-Syuan Wang
Date：2015/02/11

2. Outline
• Introduction
• Novel voice encryption method
• Principle of RPE-LTP Vocoder
• Novel Voice Encryption Method
• Encryption Algorithm
• Hardware Module
• Performance
• Conclusion

3. Introduction
• Common products mostly include:
• Cryptophone from GMSK in Germany
• Snapcell for mobile phone encrypt module
from Snapshield in Israel
• GSM Voice encrypt module from CRYPTOAG
in Sweden
• NSK200 secure phone from KONGSBERG in
Norway

4. Introduction
• Common end-to-end secure voice communication
have some obvious shortages:
• 1. Data channel connection needs several seconds
• 2. Voice processing delay in the communications
• 3. Cannot be compatible to all GSM networks

5. Introduction
• The novel encryption method can penetrate the RPE-LTP
(Regular Pulse Excitation-Long Term Prediction) vocoder,
and has the advantages of short time delay.
• RPE-LTP(Regular Pulse Excitation-Long Term Prediction):
• Reduce the amount of data, Vector Quantization…
• GMSK(Gaussian Filtered Minimum Shift Keying)

6. Principle of RPE-LTP Vocoder
• It includes pretreatment, LPC analysis, short-term
analysis filtering, long-term prediction and Regular Pulse
Excitation sequence coding
• 1. Samples digital voice signal at 8kHz sampling rate
• 2. Removes the direct current component
• 3. Make use of FIR filter
• 4. LPC analysis takes every 160 sample points (20ms)
• 5. Short-time analysis filter produces LPC residual signal
• 6. Remove redundancy farther coding
• 7. Outputs 260 bits coding every frame at last

7. Novel Voice Encryption Method
• This encryption method must penetrate the RPE-LTP
vocoder
• This new voice encryption method is a kind of signal
source encryption technology

8. Encryption Algorithm
• Provide M is plaintext, C is cipher text, k is the key, Pk is
encryption matrix generated by k, Qk is the decryption
matrix
• The algorithm is mainly intend to make the encrypted
voice signal be similar to the human natural voice signal
and can penetrate the RPE-LTP vocoder

9. Hardware Module
• The voice signal enters into the hardware module through
SPI interface. And it put back into the mobile phone
through the SPI to the RPE-LTP module

10. Performance
• The test to compare the influence on the algorithm
performance with different frame number n is based on
the platform of LeCroy 6100A 1GHZ Oscilloscope.

11. Performance
• Encryption Intensity and Residential Intelligibility
• Residential Intelligibility:

12. Performance
• Frequency domain distortion comparisons
• The recovery voice by this means has a little noise
compared with the original voice, and it doesn't affect the
whole hearing effect at all.

13. Conclusion
• This end-to-end voice encryption method is based on voice
channel
• Average processing time delay is less than 0.8s
• Penetrating to RPE-LTP compression code
• Good compatibility to GSM networks
• Suitable implementation without any change in current GSM
signaling system
• The new encryption method solves the problem that traditional
encryption algorithm cannot be used in voice channel directly
because of RPE-LTP vocoder in GSM system