Modifications of Key Schedule Algorithm on RECTANGLE Block Cipher

Abstract

Key schedule algorithm is one of the core elements that significantly affect the security of an encryption algorithm. While its importance is undeniable, the key schedule algorithm has not been given comprehensive attention compared to the encryption algorithm. RECTANGLE block cipher is very efficient in terms of encryption speed performance among the existing lightweight algorithms. However, its non-robust round keys generation seems to be the weakest point of the algorithm. A robust key schedule algorithm should produce round keys with random characteristics, independent, and not correlated to one another as defined in the randomization and confusion properties. Therefore, the objective of this paper is to improve the RECTANGLE key schedule algorithm to increase its randomization and confusion properties against high correlation keys as well as the speed and throughput performances. Three experiments were conducted based on the randomness, key sensitivity, and performance tests. The results show that our modified designs have produced lower correlation keys by 0.16% to 0.45% improvement, more random ciphertext with an increase of 13.34% to 20.00% passing rate, and better performance that recorded 1.30% to 7.82% faster and increased by 1.33% to 8.50% throughput than the original RECTANGLE.