Joint encryption and compression is an ideal solution for protecting security and privacy of image data in a real scenario, e.g. storing them on an existing cloud-based service like Facebook. Recently, some block-wise encryption-then-compression (ETC) schemes compatible with JPEG were proposed to provide a reasonably high level of security without compromising compression ratio much. This paper investigates recovering the block-wise relationship in an ETC scheme exerting on single-color blocks of size $8\times 8$ in the scenarios of ciphertext-only attack, known-plaintext attack and chosen-plaintext attack. Then, the attacking targets are extended to the other conventional ETC schemes exerting on multiple color channels and blocks of various sizes. Especially, an elaborate jigsaw puzzle solver is designed to recover enough visual information from multiple cipher-images encrypted by the same secret key. Moreover, the nice attacking performance was verified over two social media platforms, Facebook and Weibo.

翻译：联合加密与压缩是现实场景（例如将图像数据存储于Facebook等现有云服务）中保护图像数据安全与隐私的理想方案。近期，一些与JPEG兼容的块级加密-压缩（ETC）方案被提出，这些方案在不显著牺牲压缩率的前提下提供了相当高的安全级别。本文研究在唯密文攻击、已知明文攻击和选择明文攻击场景下，针对大小为$8\times 8$的单色块进行处理的ETC方案中恢复块级关系的方法。随后，攻击目标被扩展至其他基于多颜色通道及不同大小块的常规ETC方案。特别地，本文设计了一种精巧的拼图解算器，用于从由相同密钥加密的多个密文图像中恢复足够的视觉信息。此外，该攻击方法在两个社交媒体平台（Facebook和微博）上取得了良好的攻击性能验证。