Nowadays, security in data transfer is of special importance. Images are of the most attractive kinds of data in the encryption domain. Color images are more attractive than the gray-level images due to provision of more information. In the present study, various existing color (RGB mode) image encryption schemes have been examined comprehensively based on the application domains in addition to summarizing over 50 studies in this field, most of which being published in the last year. In addition, in this study, color image encryption has been categorized into ten schemes, then the proposed schemes have been compared and their advantages and limitations have been highlighted. Moreover, a complete list of common security analysis techniques for (gray or color) image encryption has been discussed which are capable of evaluating the method potential resistance to different possible attacks. The present study has been carried out to provide detailed knowledge regarding the existing image encryption schemes in the area of the RGB images. Finally, in the current study, various open issues and research directions have been considered in order to explore the promising areas for future developments.
Data transmission on various communication networks has led to the sensitivity of the security of multimedia data. Previously, text encryption was conducted through methods such as RSA (developed by Rivest, Shamir and Adleman), Data Encryption Standard (DES), Advanced Encryption Standard (AES), and International Data Encryption Algorithm (IDEA), however image encryption differs from text encryption as some intrinsic characteristics including high redundancy or bulky data capacity are merely present in images . In addition, text encryption methods lacked sufficient speed and power in diffusing and shuffling the image data . Digital images carrying a huge volume of information account for large proportion . Overall, there are two main approaches exploited to protect digital images: information hiding including watermarking, anonymity, steganography, and cover channel, and encryption including conventional encryption and other methods such as chaotic encryption .
Mehra et al. [5–7] presented a modified fusion technique in wavelet transform domain. In their proposed method, low and high frequency components are merged together to improve the image content. Also unlike optical asymmetric encryption technique based on phase- and amplitude-truncation approach, it generated four asymmetric keys corresponding to each image.
6. Conclusion and future directions
Security of images has become an important agenda of the present era. Network and communication technologies provide several modes for transferring images worldwide. Images are frequently used in various fields including online learning, engineering services, defense services, scientific experiments, medical imaging, art exhibition, or advertising. With the increased use of digital techniques for transmitting and storing images, the fundamental issue of protecting images for confidentiality, integrity, authentication, and non-repudiation are of major concerns. Since the color images carry richer information compared to the gray-level ones, color image security has become an important issue in the field of information security. Several approaches have been proposed for encrypting images. The color image encryption schemes have been outlined and examined in the present study in terms of usage. Several image encryption techniques have been proposed in the literature. Moreover, a complete list of the common security analysis techniques was presented in the current study for (gray or color) image encryption and discussed, and the potential resistance of the methods to different possible attacks was assessed. Moreover, a complete comparison of these proposed schemes was provided, which can be very useful in designing the future image encryption algorithms for color images.
The ongoing direction in the color image encryption is designing a secure method with the least possible factors for resistance against the security attacks and inclusion of the ideal properties specified in Section 5. Furthermore, considering the privacy of the secret keys is very important in image encryption; however in all of the studies employing the proposed schemes, various methods have been used for scrambling the secret keys. Therefore, designing secure color image encryption should be taken into account in the future investigations. In addition, the use of evolutionary and chaotic methods is highly necessary in all encryption applications, so that they need to be integrated with other schemes to compensate for their deficiencies.