GSM Security Threats and Countermeasures

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

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

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

4. 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 eachframe

5. A5/1 LFSRs Consists of 3 LFSRs of different lengths • 19 bits • x18 + x17 + x16 + x13 + 1 • clock bit 8 • tapped bits: 13, 16, 17, 18 • 22 bits • x21 + x20 + 1 • clock bit 10 • tapped bits 20, 21 • 23 bits • x22 + x21 + x20+ x7 + 1 • clock bit 10 • tapped bits 7, 20, 21, 22

6. A5/1 - Clocking

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

8. Design Vulnerability • Design of Clock Controlling Unit • Linear Combination Function

9. Possible Attacks • Chosen plain Text attacks • Time memory trade off attacks • Correlation attacks 

10. Proposed Counter Measures • Hardware Enhancement • Software Application- Additional Encryption

11. Hardware Enhancement

12. 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

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

14. 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.

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

16. 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.

17. Questions ????

18. Thank you all