WPE-MO: Prediction Error-Propagated Encryption with Modulo Operator for JPEG Texture Protection

We propose a JPEG format-compliant encryption method in the quantized discrete cosine transform (QDCT) domain for texture protection, called Prediction Error-Propagated Encryption with Modulo Operator (PEPE-MO = WPE-MO, by pronouncing ‘W’ as ‘double’). In the QDCT domain, both the direct current (DC) coefficients, which contain structure information, and alternating current (AC) coefficients, which contain texture information, are encrypted with newly placed prediction, encryption, and reconstruction modules. The resulting propagated prediction error reinforces texture protection. To ensure JPEG compatibility, WPE-MO incorporates a modulo operator into the prediction and reconstruction modules, circulating coefficients within the JPEG-encodable value range. Additionally, to balance attack resilience and coding efficiency, two adjustable parameters are introduced: random length interval (RLI) and random step size (RSS). Experiments on JPEG image encryption demonstrate that WPE-MO exhibits high attack resilience with minimal degradation in coding efficiency. In particular, WPE-MO resists ciphertext-only attacks, including brute-force and replacement attacks, with approximately 19.55 % degradation in coding efficiency, as measured by the Bjøntegaard-delta rate, through careful selection of RLI and RSS.