Cryptanalysis of Four-Rounded DES Using Ant Colony Optimization

It is hard for the cryptanalysts to apply traditional techniques and brute-force attacks against feistel ciphers due to their inherent structure based on high nonlinearity and low autocorrelation. In this paper, we propose a technique for the cryptanalysis of four-rounded Data Encryption Standard (DES) based on Binary Ant Colony Optimization (BACO). A known plaintext attack is used to recover the secret key of the DES cipher. The environment for the ants is a directed graph, which we call search space, is constructed for efficiently searching the secret key. We also develop a heuristic function which measures the quality of a constructed solution. Several optimum keys are computed over different runs on the basis of routes completed by the ants. These optimum keys are then used to find each individual bit of the 56 bit secret key used by DES. The results of our experiments show that ACO is an effective technique for the cryptanalysis of four-rounded DES. To the best of our knowledge, this is the first time that BACO has been used for this specific problem.