Detection of non-aligned double JPEG compression with estimation of primary compression parameters

In this paper, we propose a simple yet reliable method to detect the presence of non-aligned double JPEG compression (NA-JPEG). The method is based on a single feature which depends on the integer periodicity of the DCT coefficients when the DCT is computed according to the grid of the previous JPEG compression. Even if the proposed feature is computed relying only on DC coefficient statistics, a simple threshold detector can classify NA-JPEG images with improved accuracy with respect to existing methods and on smaller image sizes. Moreover, the proposed method is able to accurately estimate the quantization step and the grid shift of the primary JPEG compression, which can be used to perform a more detailed analysis of possibly forged images.