Square-like attacks on reduced rounds of IDEA

In this paper we develop two new chosen plaintext attacks on reduced rounds of the IDEA block cipher. The attacks exploit the word structure of the algorithm and are based on the observation that suitable chosen plaintexts give rise to some special kind of distributions which provide a way to distinguish reduced round IDEA output from a random permutation with very few plaintexts. As a result, we develop an attack for 3.5 rounds of IDEA which requires only 103 chosen plaintexts. We have reduced the number of required plaintexts significantly up to 4 rounds. We also present some interesting properties of the reduced round variants of the cipher which have not been published before. The properties and the attacks bring a different approach to analyse the cipher.