NOVEL END-TO-END VOICE ENCRYPTION METHOD IN GSM SYSTEM

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

Advisor : Dr. Jenq-Shiou LeuStudent： Bing-Syuan WangDate： 2015/02/11

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

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

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

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)

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

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

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

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

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.

Performance• Encryption Intensity and Residential Intelligibility

• Residential Intelligibility:

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.

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