CLOCK CONTROL SEQUENCE RECONSTRUCTION IN NOISY GENERATORS WITH IRREGULAR CLOCKING

摘要

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 reconstruction 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 attack scenario. The reconstruction is performed by a directed depth-first like search through the edit distance matrix. The attack is effective even if the noise level is relatively high.