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In this paper, we firstly evaluate the resistance of the reduced 5-round version of the block cipher CIKS-1 against linear crypt-analysis(LC). A feature of the CIKS-1 is the use of both Data-Dependent permutations(DDP) and internal key scheduing which consist in data-dapendent transformation of the round subkeys. Taking into account the structure of CIKS-1 we investigate linear approximation. That is, we consider 16 linear approximations with p = 3/4 for 16 parallel modulo 2² additions to construct one-round linear approximation and derive one-round linear approximation with the probability of P = 1/2 + 2^-17 by Piling-Up lemma. Also we estimate that the P is a valid probability of one-round approximation and achieve that the probability P for one-round approximation is better than 1/2 + 2^-17 through experiments. Then we construct 3-round linear approximation with P = 1/2 +2^-17 using this one-round …