GSM Security Threats and
Countermeasures
Saravanan Bala
Tanvir Ahmed
Samuel Solomon
Travis Atkison
Outline
 Introduction
 A5/1 algorithm
 Security & attacks
 Proposed Solution
 Hardware Enhancement
 Software Application
 Conclusion
Mobile Communication-GSM
 Most widely used cellular technology
 Cryptographic Algorithms- A5/1, A5/2, A5/3
 A5/1 Algorithm provides over the air privacy
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
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
A5/1 - Clocking
A5/1 clocking
 Majority rule
 m=maj(c1, c2, c3)
 m=maj(1, 1, 0)
 maj = 1
 Registers R1 & R2
Design Vulnerability
 Design of Clock Controlling Unit
 Linear Combination Function
Possible Attacks
 Chosen plain Text attacks
 Time memory trade off attacks
 Correlation attacks
Proposed Counter Measures
 Hardware Enhancement
 Software Application- Additional Encryption
Hardware Enhancement
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
Contd..
 Linear combining functions are cryptographically weak
functions
 Non Linear Combining Function
 Combining function not fixed - changed dynamically by
using a 2:1 multiplexer.
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.
CONCLUSION
 Proposed scheme generates cryptographically better key
sequence than the current version of A5/1
 Future mobile communications can be handled using UMTS
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.
Questions ????
Thank you all