Solving the Block-Wise Puzzles in an Encryption-Then-Compression System

Joint encryption and compression provide an effective solution for securing image data, which is crucial for transmission and storage on cloud services such as Facebook and Twitter. Recent advancements in block-wise encryption then-compression (ETC) schemes, particularly those compatible with JPEG, strike a balance between high security and efficient compression in the application scenarios. This paper analyzes the interactions within ETC schemes applied to single-color 8×8 blocks under various attack scenarios, including ciphertext only, known-plaintext, and chosen-plaintext attacks. Then, the analysis object is further extended to conventional ETC schemes involving multiple color channels and blocks of different sizes. Notably, we introduce an advanced jigsaw puzzle solver designed to reconstruct the relative positions of sufficient blocks in the original plain-image from multiple cipher-images encrypted with the same secret key. The effectiveness of this solver is demonstrated through tests conducted on popular social media platforms such as Facebook and Weibo, underscoring its potential for real-world applications.