# The Design and FPGA-Based Implementation of a Stream Cipher Based on a Secure Chaotic Generator
## 1 Introduction  
The protection of information against unauthorized eavesdropping and exchanges is essential, in particular for military, medical, and industrial applications. Nowadays, cryptographic attacks are more and more numerous and sophisticated; consequently, new effective and fast techniques of information protection have appeared or are under development. In this context, recent works have focused on designing new chaos-based algorithms, which provide reliable security while minimizing the cost of hardware and computing time. Chaos theory was first discovered in the computer system by [PERSON] in 1963 [1]. A chaotic system, although deterministic and not truly random, has unpredictable behavior, due to its high sensitivity to initial conditions and control parameters which constitute the secret key. It can generate an aperiodic analog signal whenever its phase space is continuous (i.e., with an infinity of values). However, when its phase state is discrete (with a finite set of values), its orbits must be periodic, even with a very long period. In the field of chaos-based digital communication systems, the chaotic signal has been one of the main concerns in recent decades and is widely used to secure communication. In chaos-based cryptography, discrete chaotic maps are used in most chaotic systems (encryption, steganography, watermark, hash functions) to generate pseudo-random chaotic sequences with robust cryptographic properties [2, 3, 4, 5, 6, 7, 8, 9, 10]. In a stream cipher, the pseudo-random number generator (PRNG) is the most important component since all the security of the system depends on it. For this, a new category of pseudo-chaotic number generator(PCNG) has been recently built to secure stream-data [11; 12; 13; 14]. These PCNGs use combined chaotic maps because single chaotic maps are not secure for use in stream ciphers. In 2017, [PERSON] et al. [15] designed a novel stream cipher based on an efficient chaotic generator; the results obtained from the cryptographic analysis and of common statistical tests indicate the robustness of the proposed stream cipher. In 2018, [PERSON] et al. [16] developed two new stream ciphers based on pseudo-chaotic number generators (PCNGs) that integrate discrete chaotic maps and use the weak coupling and switching technique introduced by [PERSON] [17]. Indeed, the obtained results show that the proposed stream ciphers can be used in practical applications, including secure network communication. In 2019, [PERSON] et al. [18] proposed a new lightweight stream cipher system based on chaos--a chaotic system--and two nonlinear feedback shift registers (NFSRs) are used. The results show that the stream cipher has good cryptographic characteristics. In 2020, [PERSON] et al. [19] proposed several efficient multimedia encryption techniques based on four combined chaotic maps (Arnold Map, Lorenz Map, Chebyshev Map, and logistic Map) using serial or parallel connections. With the rapid growth of Internet of Things (IoT) technology that connects devices with low power consumption and low computing resources, the hardware implementation of chaotic and non-chaotic ciphers is more suitable than a software implementation. Note that few chaotic encryption systems are realized in the hardware [20; 21; 22]. In this study, we designed an efficient chaos-based stream cipher (SCbSC) using a proposed secure PCNG. Then, we addressed the hardware implementation and evaluated the performance in terms of resilience against cryptanalytic attacks and in terms of hardware metrics (areas, throughput, and efficiency). The proposed system uses three weakly coupled chaotic maps (3D Chebyshev, logistic, and skew-tent) and integrates a masking technique in the recursive cells to resist side-channel attacks (SCAs). Its implementation on a Xilinx XC7Z020 PYNQ-Z2 FPGA hardware platform achieves a throughput of 1.1 Gbps at an operating frequency of 37.25 MHZ.