I. Introduction
In smart cities, information security plays a major role in protecting the higher levels of confidentiality, availability, and integrity as well as the stability that national services and organizations need to support sustainable and livable smart environments. Cryptographic algorithms are used for security services in various environments in which less number of hardware resources used, high speed and power consumption are key requirements. The Advanced Encryption Standard (AES), also known as Rijndael is an encryption algorithm developed by Dr. Joan Daemen and Dr. Vincent Rijmen [1]. In 2001, it was adopted as a Federal Information Processing Standard by the National Institute of Standards and Technology [2]. S-box substitution is at the core of any AES implementation. It is the only non-linear and complex step in each round of encryption algorithm. There are different implementations of AES S-box, among them, three main. The first, by direct mapping from lookup table (LUT) which stores all predefined 256 8-bit values of S-box in a Read-Only-Memory (ROM) [3], [10], [12]–. The second, using composite field arithmetic in which the field operations are implemented in lower order fields and by lower cost subfield operations [9], [11] and the third by using combinational logic only, to avoid the unbreakable delay of LUTs and to achieve any further increase in processing speed [6]–[8]. In this article, S-box implementation using combinational logic is adopted.