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GSM Security Threats and Countermeasures Saravanan Bala Tanvir Ahmed Samuel Solomon Travis Atkison

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Outline Introduction A5/1 algorithm Security & attacks Proposed Solution Hardware Enhancement Software Application Conclusion

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Mobile Communication-GSM Most widely used cellular technology Cryptographic Algorithms- A5/1, A5/2, A5/3 A5/1 Algorithm provides over the air privacy

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A5/1 GSM phone conversations: sequences of frames. One 228 bit = frame is sent in 4.6 milliseconds: 114 bits for the communication in each direction. A5/1 produces 228 bits to XOR with the plaintext in each frame

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A5/1 LFSRs Consists of 3 LFSRs of different lengths 19 bits x 18 + x 17 + x 16 + x clock bit 8 tapped bits: 13, 16, 17, bits x 21 + x clock bit 10 tapped bits 20, bits x 22 + x 21 + x 20 + x clock bit 10 tapped bits 7, 20, 21, 22

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A5/1 - Clocking

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A5/1 clocking Majority rule m=maj(c1, c2, c3) m=maj(1, 1, 0) maj = 1 Registers R1 & R2

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Design Vulnerability Design of Clock Controlling Unit Linear Combination Function

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Possible Attacks Chosen plain Text attacks Time memory trade off attacks Correlation attacks

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Proposed Counter Measures Hardware Enhancement Software Application- Additional Encryption

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Hardware Enhancement

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Contd.. Enhanced Majority Rule Computes two majority values m1=maj(b1, b2, b3) m2=maj(c1, c2, c3) Let S1 = { } and S2 = { } (Imaginary sets) S1∩S2

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Contd.. Linear combining functions are cryptographically weak functions Non Linear Combining Function Combining function not fixed - changed dynamically by using a 2:1 multiplexer.

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Software Application End to end encryption Encrypt speech signal at user end Solution includes using transmission of encrypted voice GSM Data Call CSW Example : SecureGSM Another solution includes usage of connection based packet switching. Example : Babylon nG Both techniques use Diffie-Hellman key agreement protocol for ciphering key exchange and AES cipher for encryption of voice. Experimentally proved that the implementation of AES cipher provides more robust and efficient system.

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CONCLUSION Proposed scheme generates cryptographically better key sequence than the current version of A5/1 Future mobile communications can be handled using UMTS

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REFERENCES [1] “Secure Mobile Communication Using Low Bit-Rate Coding Method”. IEEE paper published by Wasif, M.; Sanghavi, C.R.; Elahi, M.; [2] “Another attack on A5/1”. IEEE paper published by Patrik Ekdahl and Thomas Johansson. [3] “Enhanced A5/1 Cipher with Improved Linear Complexity”. IEEE paper published by Musheer Ahmad and Izharuddin. [4] “Introduction to the design & analysis of algorithms” by Anany Levitin. [5] Based on the presentation given by Karsten Nohl on the “26th Chaos Communication Congress (26C3)” conference. [6] “Security Enhancements in GSM Cellular Standard”. IEEE paper published by Musheer Ahmad and Izharuddin. [7] “Communication Security in GSM Networks” published on 2008 international conference on security technology by Petr Bouška, Martin Drahanský. [8] “Implementation and Analysis of AES, DES and Triple DES on GSM network” an IEEE paper published by Sachin and Dinesh kumar [9] “Construction of nonlinear Boolean functions with important Cryptographic properties - Advances in Cryptology” by Sarkar and Maitra. [10] Diagram in slide number 6 taken from wikipedia.

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Questions ????

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Thank you all

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