**2. Related works**

In this section, we recall diverse related works that studied the encryption algorithms designed for securing digital images.

An image cryptosystem that combines chaos sequences and a modified AES algorithm is put forward in [1]. Firstly, the key is generated by Arnold's chaos sequence. Secondly, the plain image is ciphered by the modified AES and by implementing the round keys produced by the chaos system. In [2], the authors proposed a dynamic AES for numerical image encryption based on the logistic chaotic map and the advanced encryption standard (AES) with Galois mode. Utilizing the logistic mapping, the key is generated and mixed at different stages with the plain data. In [3], the authors put forward a secure framework using a chaos system-based S-box. However, a chaos system is employed to generate three sequences of random numbers, then, an S-box is performed. The image is ciphered by the XOR operation and the sub-byte function.

The main issue of these works is that they suffer from the secure transmission of the symmetric key. In [4], a secure method for color image protection is suggested. It is based on the elliptic curve and AES. Random numbers are generated by the elliptic curve, hence these numbers are employed for generating three maskers to cipher the three components-red, blue, and green-of the image. The drawback of this paper is

*Hardware Implementation of an Improved Hybrid Cryptosystem for Numerical Image Encryption… DOI: http://dx.doi.org/10.5772/intechopen.105207*

that all operations are performed sequentially, which can degrade the system. In [5], the authors present a novel idea to protect images against attacks. The initial level is performed by applying the conformal mapping to the private data. Then the image result is encrypted and decrypted using the (RSA) algorithm. Thirty they use less significant bit (LSB) as the hiding method to hide the message inside the cover image. Finally, they propose to compress the image using GZIP. However, the use of an asymmetric algorithm to encrypt big data can degrade the system in terms of execution time. In [6], the authors propose a quantum logistic image cryptosystem that combines both RSA and SHA-3 algorithms. Firstly, the RSA is employed to randomly generate key pairs with private keys and public keys. A fixed matrix is then generated for the confusion. Secondly, the preprocessed image is calculated by the hash function SHA-3 to obtain the clear message that is then stored safely. Using the RSA algorithm, the encrypted message can be performed corresponding to the original message. After mixing both the original and the encrypted messages, the initial conditions of the quantum logistic map are computed using a novel mathematical technique. The main drawback of this paper is the use of an asymmetric algorithm that is a hard algorithm to encrypt the big data.

Our motivation for this work is to guarantee all services of cryptography with low computational time.
