# Design, Implementation, and Analysis of a Block Cipher Based on a Secure Chaotic Generator
## 1 Introduction  
Nowadays, the necessity to protect data has become a primary challenge. The security of communication of sensitive data is not only limited to military and diplomatic information and national defense issues, but also concerns the private lives of individuals and companies. With this regard, cryptography schemes have been developed. Indeed, cryptographic techniques leads to well-ensured messages and data confidentiality. Chaos was integrated into cryptography for the first time by [PERSON] in the 1990s [1]. Among the existing state-of-the-art approaches, chaos-based cryptosystems have proved their efficiency in data security [2; 3; 4] due, on the one hand, to the powerful proprieties of chaotic sequences such as deterministic, unpredictable, random-like behavior, and high sensitivity to initial conditions and control parameters (secret key) [5; 6; 7] and, on the other hand, to their efficient properties of confusion and diffusion performed by chaotic maps [8; 9; 10; 11; 12]. Symmetric chaos-based encryption/decryption systems are classified under two categories: block ciphers and stream ciphers. The former is the main symmetric key cryptosystem, as their design is related to [PERSON]'s theory of information security based on confusion and diffusion operations [8; 13]. Their security properties were well-studied, and these ciphers encrypt the plaintext on a block of bits: 128, 256, 512, 1024, etc. [14; 15]. Compared to traditional symmetric cryptography, chaos-based encryption/decryption schemes are more flexible (generic design, variable block size, and secret key can be easily enlarged), easier to implement, and have intrinsic security related to the properties of chaotic sequences [14; 16]. Moreover, some of them use a substitution layer based on keydependent S-boxes and not on fixed S-boxes as used in the Advanced Encryption Standard (AES) algorithm [17]. A key element of all chaos-based cryptosystems is the chaotic generator, which provides the dynamic keys (encryption keys) for the confusion and diffusion layer. The security, as well as the efficiency of the system, largely depend on the chaotic generator used. However, most chaos-based cryptosystems reported in the literature refer to simple chaotic maps to supply the confusion and diffusion layer [18; 19]. Such cryptosystems can be destroyed by side-channel attacks [20; 21]. A fast chaos-based image encryption system with a dynamic state variables selection mechanism was proposed by [PERSON] et al. [22] and it achieved a high confusion and diffusion process. Similar to the suggested cryptosystem by [PERSON] [9] and [PERSON] et al. [23], three prototypes of a chaos-based cryptosystem were put up by [PERSON] et al. [24]. The modified 2-D cat map was used to create the confusion process and 32-bit and 8-bit logistic maps were used as the diffusion layer for the two first versions, and a modified finite skew tent map (FSTM) in the third version. A secure cryptosystem based on chaotic components was proposed by [PERSON] et al. [25]. In this cryptosystem, the AES S-box was used as a confusion layer. [17]. A global diffusion operating on the entire image was established using a horizontal addition diffusion (HAD), afterward, a vertical addition diffusion (VAD), introduced by [PERSON] et al. [26], then a permutation operation based on the modified 2-D cat map to reinforce the diffusion effect. [PERSON] et al. [27] published a block cipher cryptosystem using dynamic S-boxes based on a tent map. Their system operated on blocks using different generated S-boxes and used 32 iterations of substitution and left cyclic shift. A similar cryptosystem was proposed by [PERSON] et al. [28] based on a circular S-box as a confusion process and a keystream buffer as a diffusion layer. The system operates on the entire plain image. Another block cipher based on the chaotic logistic map was presented by [PERSON] et al. [29]. To enhance the performance of the logistic map, a new chaotification approach based on a multiplicative inverse function was applied.