# Grayscale-Based Image Encryption Considering Color Sub-sampling Operation for Encryption-then-Compression Systems

## TL;DR

This paper introduces a grayscale-based image encryption scheme for EtC systems that improves security and compression efficiency by enabling smaller blocks, more blocks, and color sub-sampling, even with grayscale images.

## Contribution

The proposed scheme allows smaller block sizes, more blocks, and the use of color sub-sampling in encrypted images, enhancing security and compression in EtC systems.

## Key findings

- Enhanced security against jigsaw puzzle and brute-force attacks.
- Maintains high compression performance with color sub-sampling.
- Effective in real-world social media transmission scenarios.

## Abstract

A new grayscale-based block scrambling image encryption scheme is presented to enhance the security of Encryption-then-Compression (EtC) systems, which are used to securely transmit images through an untrusted channel provider. The proposed scheme enables the use of a smaller block size and a larger number of blocks than the conventional scheme. Images encrypted using the proposed scheme include less color information due to the use of grayscale images even when the original image has three color channels. These features enhance security against various attacks, such as jigsaw puzzle solver and brute-force attacks. Moreover, it allows the use of color sub-sampling, which can improve the compression performance, although the encrypted images have no color information. In an experiment, encrypted images were uploaded to and then downloaded from Facebook and Twitter, and the results demonstrated that the proposed scheme is effective for EtC systems, while maintaining a high compression performance.