A Chosen Plaintext Linear Attack on Block Cipher CIKS-1

Abstract

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 approximation and can attack the reduced 5-round CIKS-1 with 64-bit block by LC. In conclusion, we present that our attack requires about 2³⁶ chosen plaintexts with a probability of success of 78.5% and 1/5 × 2³² × 2³⁶ ≈ 2^65.7 encryption times to recover last round(5-round) key. In addition, we discuss a few improvements of the cipher CIKS-1.