AC Prediction Error Propagation-based Encryption for Texture Protection of JPEG Compressed Images

A recent attention of JPEG format-compliant encryptions has been concentrated to randomize the specific syntaxes of JPEG file interchange format (JFIF) or to flip the signs of quantized discrete cosine transform (QDCT) coefficients. Although the random sign flip (RSF) achieves low-complexity encryption while preserving the JPEG coding efficiency, the texture information represented by nonzero AC coefficients are still easy to reveal for an attacker using replacement attacks. To reinforce the perceptual degradation and the texture protection at the same complexity as the RSF, we propose an AC prediction error propagation-based encryption (AC-WPE) in the JPEG QDCT domain. Since the AC-WPE encrypts the coefficient signs in the residual QDCT domain obtained by a provisional AC predictor such as differential pulse-code modulation (DPCM) and propagates the encrypted prediction errors among the several blocks by the AC reconstructor, the attack resilience is obviously reinforced. Moreover, to suppress the decrease in coding efficiency, we introduce the randomized prediction intervals into the AC predictor/reconstructor. Experiments with JPEG encryption show that the AC-WPE reinforces the perceptual degradation and the resilience to the replacement attacks, while reasonably expensing the bitrate overheads of crypto-compressed images.