Improved square properties of IDEA

Block cipher encryption algorithms generally process on word structures of fixed length such as 8 or 16bits. IDEA is one of the most widely used block ciphers and operates on 16-bit words. Square analysis is a method that exploits the word structure of block ciphers. Some square distinguishers of IDEA are given in previous studies. The best attacks against IDEA use square-like techniques. In this paper, we focus on the square properties of the IDEA block cipher. We consider all fixed word combinations of the plaintext to investigate the structural behavior of the algorithm. We observe that the cipher can be distinguished from a random permutation by fixing one, two or three subblocks of the cipher for 2 and 3 rounds. We find out novel 3-round distinguishers that require 2 chosen plaintexts. Furthermore, this approach enables us to propose the first four and five round square distinguishers of IDEA.