Visually Semantics-Aware Color Image Encryption Based on Cross-Plane Substitution and Permutation

The concept of thumbnail-preserving encryption (TPE) is proposed to strike a balance between image privacy and visual usability. It allows users to have both without having to choose between privacy and usability, but it processes the information within a channel and not between channels. Recently, two cross-plane TPE schemes have been proposed in an attempt to process the information between channels. However, in a sense, they are not completely cross-plane. In addition, as with other TPE schemes, they do not respect the syntax of the color image. They simply treat each channel as a regular numerical matrix for the substitution encryption, and thus there are issues of out of bounds and some elements being left unencrypted. In this article, we propose a TPE scheme for cross-plane substitution and permutation. As with other TPE schemes, a certain amount of visually semantics-aware information is preserved in the encrypted image. Meanwhile, this TPE scheme is the first to implement a color image compliant syntax. In general, a color image composes of three channels, meaning a (pixel) point has three elements. In the proposed scheme, substitution encryption can directly be used for a point (three elements across planes). It eliminates possibly going out of bounds and ensures that all elements in the image are encrypted during substitution.