Compressing JPEG compressed image using reversible data hiding technique

Since the concept of reversible data hiding technique was introduced, many researchers have applied it for authentication of uncompressed images. In this paper, an algorithm is introduced to compress JPEG files again without any loss in image quality. The proposed method can modify an entire segment of VLC codeword sequence to embed a bit of data. The modified codewords may destroy the correlation, or the smoothness, between neighboring pixels of the recovered image. The data extractor utilizes the smoothness change to know the hidden data. For this, a novel smoothness measurement function which uses both inter- and intra-MAD values is proposed. When the smoothness change is small, two consecutive segments are concatenated to extract correct data with higher smoothness sensitivity. As a result, compression ratio or embedding capacity is increased in most natural images.