Joint image compression and encryption based on alternating transforms with quality control

In this paper, we propose a novel joint image compression and encryption technique where an annoying image can still be recovered even without the encryption key. Our work is based on JPEG standard. By embedding encryption algorithm at the transformation stage, we realize image encryption and compression together with controllable image quality. Instead of using the 8×8 discrete cosine transform (DCT) alone for transformation, we develop new orthogonal transforms by introducing sign-flips into the butterflies of DCT's flow-graph structure, and then employ them alternatively in JPEG's transformation stage according to a secret key. By carefully selecting the butterflies for sign-flipping, we can control the visual quality of the encrypted images. Finally, a detailed security analysis of our proposed encryption algorithm is presented to show its resistance to various attacks, such as cryptographic attack, replacement attack and statistical model-based attack.