Digital Systems and Media Computing Laboratory School of Science & Technology

1. A High-Speed Hardware Implementation of the LILI-II Keystream GeneratorParis Kitsos ...in cooperation with Nicolas Sklavos and Odysseas Koufopavlou Digital Systems and Media Computing Laboratory School of Science & Technology Hellenic Open University, Patras, Greece e-mail: pkitsos@ieee.org

2. Presentation Overview • LILI-II specifications overview • Proposed hardware architecture • VLSI implementation results • Comparisons in terms of FPGA area, clock frequency and performance with existing works

3. LILI-II Specifications (I) • LILI-ΙΙ generator is a clock-controlled nonlinear filter generator • LILI-II • Use two binary LFSRs and • Two functions in order to generate a pseudorandom binary keystream sequence • The components of LILI-II grouped into two subsystems, based on the functions they perform • Clock control and Data generator

4. LILI-II Specifications (II) • The LFSR for the clock-control subsystem is regularly clocked • The output of the Clock-Control LFSR control the Data-Generation LFSR

5. LILI-II Specifications (III) • Clock-control LFSR (LFSRc) • Use a primitive polynomial with length equal to 128 • The function fc defined as • Data Generation LFSR (LFSRd) • Use a primitive polynomial with length equal to 127 • The Boolean function fd has 12 inputs for the LFSRd stages and defined by a truth table

6. Proposed Architecture (I) • The proposed architecture consists of the clock-control subsystem and the data generation subsystem • Operation • Initialization phase • Keystream generation phase • Initialization Phase • Use the secret key and initialization vector and operates twice and the output feeds the LFSRs as new values • Keystream generator Phase • When the initialization phase finish the generator produce the appropriate keystream bits.

7. Proposed Architecture (II)

8. Proposed Architecture (III) • The Clock-Control subsystem is comprised by the LFSRc, the function fc and the Clock Pulses components. • The fc is a simple 3-bit adder. • The Clock Pulses control the LFSRds through the AND gates.

9. Proposed Architecture (IV) • The Data-Generation subsystem is comprised by 4 LFSRds, 4 AND gates, the function fd, 6 Pipeline Registers and 12 4x1 Multiplexers. • Pipeline registers are located in the LFSRd(i) outputs in order to equalise the data delays between of them • The multiplexers (MUXs) are used in order to combine the appropriate LFSRds positions • The fd function is implemented by ROM with 4096 per 1-bit elements

10. A Different Approach • Many applications, in the same device, demand different security levels. This could be achieved with the usage of reconfigurable LFSRs, if different feedback polynomials selected any time.

11. VLSI Implementation Results

12. Comparisons

13. Conclusions • An efficient hardware implementations of the LILI-II keystream generator was presented • Achieves a throughput equal to 366 Mbps • The proposed architecture is more hardware efficient than previous works

14. Questions ?