# Chaotic Image Encryption: State-of-the-Art, Ecosystem, and Future Roadmap
## 1 Introduction  
Image processing is used in various computing environments [1; 2]. Image processing techniques take advantage of different security mechanisms. Among these mechanisms, in this paper, we focus on encryption, which has been of critical importance in image processing [3], as well as many other areas [4; 5; 6].  
In recent years, the cryptography research community has taken advantage of the advancements in different technologies and theories including information theory [7], quantum computing [8], neural computing [9], Very Large Scale Integration (VLSI) technology [10], and especially, chaos theory [11].  
All the above-mentioned theories have especially affected image encryption. However, in this paper, we are specifically interested in the applications of chaos theory in image encryption. Chaos is the characteristic of a system whose current state is guaranteed to be highly sensitive to the previous state (spatial chaos), the initial conditions (temporal chaos), or both (spatio-temporal chaos). Such a sensitivity makes the output or the behavior of a chaotic system difficult to predict. Chaos theory justifies and formulates the apparent disorder of chaotic systems on the basis of orderly patterns, structured feedback loops, iterative repetitions, self-organization, self-similarity, fractals, etc. Chaotic maps, attractors, and sequences all refer to the mathematical structures used for this formulation. Chaotic systems, maps, attractors, and sequences have been of great interest to the research community in recent years [12; 13]. They have been used for security purposes in a broad variety of applications ranging from smart grids [14] to communication systems [15]. Especially, chaotic encryption has been used for encrypting a variety of content types in addition to images [1; 2].  
Figure 1 illustrates how image encryption converges with chaos theory at chaotic image encryption.  
Figure 1 first of all introduces the icons we will use in the rest of this paper to represent _image processing_, _encryption_, _image encryption_, _chaos_, _and chaotic image encryption_. Furthermore, this figure shows how image processing joins encryption and then chaos theory to build _chaotic image encryption_ as a branch of science and a field of research.  
A comprehensive survey on research works focusing on chaotic image encryption can pave the way for further research in this area via highlighting current trends, shedding light on less-studied related topics, and developing directions for future research in this field. In addition to presenting such a survey, this paper establishes an ecosystem for chaotic image encryption. The ecosystem contains the following items.  
* **Challenges:** The problems that can potentially make chaotic image encryption difficult, costly, or challenging.
* **Applications:** Environments or areas wherein chaotic image encryption has been demonstrated to be of assistance or efficiency.
* **Enablers:** The technologies or branches of science that can support chaotic image encryption via improving its feasibility, security, performance, or cost efficiency.  
The literature comes with several surveys somehow relevant to the work of this paper. However, some of them are too outdated for such a fast-moving research area. Some existing surveys do not focus on chaotic image encryption, and others fail to develop an ecosystem for chaotic image encryption or a future roadmap for further research in this area. These shortcomings motivate our work in this paper.  
The rest of this paper is organized as follows. Section 2 studies existing surveys and their shortcomings to highlight our motivations for the work of this paper. Section 3 discusses the state-of-the-art of chaotic image encryption in chaos, image, and encryption aspects. Section 4 studies the ecosystem of chaotic image encryption. Section 5 develops the future roadmap, and lastly, Section 6 concludes the paper.  
Figure 1: Chaotic Image Encryption: The Convergence Point of Image Encryption and Chaos Theory.