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 x 18 + x 17 + x 16 + x 13 + 1 clock bit 8 tapped bits: 13, 16, 17, 18 22 bits x 21 + x 20 + 1 clock bit 10 tapped bits 20, 21 23 bits x 22 + x 21 + x 20 + x 7 + 1 clock bit 10 tapped bits 7, 20, 21, 22
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  “Secure Mobile Communication Using Low Bit-Rate Coding Method”. IEEE paper published by Wasif, M.; Sanghavi, C.R.; Elahi, M.;  “Another attack on A5/1”. IEEE paper published by Patrik Ekdahl and Thomas Johansson.  “Enhanced A5/1 Cipher with Improved Linear Complexity”. IEEE paper published by Musheer Ahmad and Izharuddin.  “Introduction to the design & analysis of algorithms” by Anany Levitin.  Based on the presentation given by Karsten Nohl on the “26th Chaos Communication Congress (26C3)” conference.  “Security Enhancements in GSM Cellular Standard”. IEEE paper published by Musheer Ahmad and Izharuddin.  “Communication Security in GSM Networks” published on 2008 international conference on security technology by Petr Bouška, Martin Drahanský.  “Implementation and Analysis of AES, DES and Triple DES on GSM network” an IEEE paper published by Sachin and Dinesh kumar  “Construction of nonlinear Boolean functions with important Cryptographic properties - Advances in Cryptology” by Sarkar and Maitra.  Diagram in slide number 6 taken from wikipedia.