Clock Control Sequence Reconstruction in Noisy Generators with Irregular Clocking

S.V. Petrović and A. Fúster-Sabater (Spain)

Keywords

Telecommunications technology, Spreadspectrum, Cryptanalysis, Irregular clocking, Edit distance.

Abstract

Clock control sequence reconstruction is a key phase in the cryptanalysis of irregularly clocked Linear Feedback Shift Registers (LFSRs), which are widely used in spread spectrum systems. The previously published reconstruc tion methods have been designed to work in the known plaintext attack scenario, i.e. without noise. However, the influence of noise on the effectiveness of the clock control sequence reconstruction is decisive. We present a clock control reconstruction procedure for the ciphertext only at tack scenario. The reconstruction is performed by a di rected depth-first like search through the edit distance ma trix. The attack is effective even if the noise level is rela tively high.

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