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The recent increase of resource-constrained embedded devices have led to the need of lightweight cryptography. Therefore, the design of secure communication algorithms that fit in this highly constrained environments has become a fundamental issue in cryptographic circuit design. In this paper, we propose an optimization methodology that would efficiently reduces the code size of the S-box, the most expensive operation of the Advanced Encryption Standard (AES). Here, we perform a study on composite field AES S-box constructed using an inversion algorithm based on Fermat's Little Theorem (FLT). Consequently, we derive two AES S-box constructions over the fields GF((24)2) and GF((22)4) respectively. Our methodology results in smaller computational cost compared to the conventional Look-up Table (LUT) method, which is commonly deployed on microcontrollers.