Ciphertext-Only Attack on Grayscale-Based EtC Image Encryption via Component Separation and Regularized Single-Channel Compatibility

TL;DR

This paper introduces a new method to attack grayscale-based image encryption systems by reconstructing encrypted images from ciphertext alone.

Contribution

A practical ciphertext-only attack is proposed using component separation and regularized compatibility measures.

Findings

The proposed attack successfully recovers recognizable image content where existing methods fail.

Texture-based classification and regularized compatibility improve reconstruction accuracy and efficiency.

The results reveal a potential security vulnerability in grayscale-based encryption systems.

Abstract

Grayscale-based Encryption-then-Compression (EtC) systems transform RGB images into the YCbCr color space, concatenate the components into a single grayscale image, and apply block permutation, block rotation/flipping, and block-wise negative–positive inversion. Because this pipeline separates color components and disrupts inter-channel statistics, existing extended jigsaw puzzle solvers (JPSs) have been regarded as ineffective, and grayscale-based EtC systems have been considered resistant to ciphertext-only visual reconstruction. In this paper, we present a practical ciphertext-only attack against grayscale-based EtC. The proposed attack introduces three key components: (i) Texture-Based Component Classification (TBCC) to distinguish luminance (Y) and chrominance (Cb/Cr) blocks and focus reconstruction on structure-rich regions; (ii) Regularized Single-Channel Edge Compatibility (R-SCEC), which applies Tikhonov regularization to a single-channel variant of the Mahalanobis Gradient Compatibility (MGC) measure to alleviate covariance rank-deficiency while maintaining robustness under inversion and geometric transforms; and (iii) Adaptive Pruning based on the TBCC-reduced search space that skips redundant boundary matching computations to further improve reconstruction efficiency. Experiments show that, in settings where existing extended JPS solvers fail, our method can still recover visually recognizable semantic content, revealing a potential vulnerability in grayscale-based EtC and calling for a re-evaluation of its security.

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