Study On Reversible Watermarking Algorithms For Halftone Images And Their Application

Being able to restore the original content of the cover image,reversible data hiding is important and widely used in fields like judicial forensics,medical diagnosis and military.In the cloud environment,users need to encrypt the data to prevent private data from leaking.While on the cloud platforms,it is needed to embed extra information directly into the ciphertext domain to manage the data through retrieving,authenticating and so on,which requires the application of reversible data hiding.The halftone images are special images which only consist of black and white pixels.Since there is little redundancy in the halftone images,it is hard to achieve completely reversible data hiding.Therefore,this thesis conducts the research on the reversible data hiding for halftone images as follows:In the plaintext domain,this thesis proposes an algorithm based on overlapping block scanning and central pixel flipping.Based on the theory about the flippability of the central pixel in a 3×3 block,this algorithm performs a raster scan of 3×3 blocks with a step size of 2 on the halftone image.It is found that there are some pairs of block patterns containing central pixels of non-zero flippability remaining unchanged before and after flipping.These blocks are defined as special blocks,and then get used for watermark embedding.The overlapping block scanning expands the embedding capacity,while the use of special blocks ensures that the image has good visual quality after embedding.In the ciphertext domain,this thesis proposes an algorithm based on secret sharing and block rearranging.Using secret sharing,this proposed algorithm converts the original halftone image into sub-images,making extra redundancy to compensate the low redundancy nature of the halftone image and enabling the algorithm to get used in multiuser application by sharing sub-images to more than one user.Moreover,this algorithm uses the encrypted sub-image as the cover image,and also encrypts the watermark before embedding,providing more security when compared to plaintext domain algorithms.Comparing to some other related algorithms,tests show that these two proposed methods both improve in embedding capacity and are reversible.These algorithms are proved to be effective,providing a solution for the halftone image reversible watermarking.