Cryptanalysts Representation of Nonlinearly Filtered ML-Sequences
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A running key generator consisting of a maximum-length (ML) linear feedback shift register (LFSR) and some nonlinear feedforward state filter function is investigated. It is shown how a cryptanalyst can find an equivalent system in a ciphertext-only attack. The analysis uses a Walsh orthogonal expansion of the state filter function and its relation to the crosscorrelation function (CCF) between the ML-sequence and the produced running key sequence.
KeywordsStream Cipher Equivalent System Equivalent Function Walsh Function Plaintext Attack
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