Abstract
Substitution plays a vital role in enhancing the security of symmetric block ciphers. Randomized substitution ismore effective in triggering of confusion in symmetric block ciphers as compared to static substitution. Mostly the existing substitution methods used in Data Encryption Standard and in standardized Advanced Encryption Standard (AES) are static in nature. However, some efforts have been made in earlier years to replace the static S-box of AES with dynamic S-box, but all these dynamic substitution approaches are not truly random in nature. Thus, existing dynamic substitution methods are based on publically known substitution transformation and are not feasible for dynamically sized block ciphers, unlike the proposed substitution method. The proposed randomized substitution method (RSM) utilizes a pseudorandom-based direct association with a secret key without having any publicly known S-box transformation. Moreover, proposed RSM does not retain any irreducible polynomial {11B} in Galois field GF(2(8)). The randomness properties of the proposed method have been evaluated through several well-known statistical tests with a standard tool (Statistical Testing Suite) recommended by the National Institute of Standard and Technology (NIST). Experimental results show that proposed RSM contains significant randomness properties which reflects the recommendations of NIST to be considered as a randomized substitution method.