Secure Steganography For Digital Image

Steganography hides secret data in normal media,and transmits the stego media via public channel to achieve covert communication.Batch steganography is able to achieve the covert transmission of a large number of secret data,which is significant to national security and privacy protection.This dissertation mainly focuses on the frontier research of steganography in digital images,and studies the steganographic theory and methods on single and multiple images.The contributions of this dissertation are listed as follows:1.On Improving Distortion Functions for JPEG SteganographyThis dissertation proposes a universal method to improve existing distortion functions designed for JPEG steganography.Given a JPEG image,a reference image close to the image before JPEG compression is constructed.Guided by the reference image,the priority of +1 and –1 embedding for each DCT coefficient is determined.Then the modification amplitude of each +1 or –1 cost value is determined by minimizing the feature distance between cover and stego image.This method can be jointed with multiple steganalytic features to significantly increase the undetectability of steganography.2.Joint Cover-Selection and Payload-Allocation by Steganographic Distortion OptimizationThis dissertation proposes a batch steganographic method which combines cover-selection and payload-allocation by steganographic distortion optimization.Firstly prove that with the value of payload increasing,the first-order derivative of steganographic distortion of a single cover is monotonically increasing.Then,it deduce that the first-order derivative of steganographic distortion of covers that selected from a given set should be equal if the total steganographic distortion of the corresponding selected covers is minimal.Finally,an algorithm is designed to combine cover-selection and payload-allocation,so that the optimal stego-covers can be obtained.As a result,the undetectability of steganography is obviously improved.3.Secure Cover Selection for SteganographyThis dissertation proposes a cover selection method for steganography which is able to resist pooled steganalysis and single object steganalysis meanwhile.To resist pooled steganalysis,the MMD(maximum mean discrepancy)distance between the stego set and a clear arbitrary image set is kept not larger than a normal threshold during cover selection,where the threshold is the MMD distance between two clear arbitrary image sets.Under this constraint,a searching strategy is designed to select the minimalsteganographic distortion images within affordable computational complexity to resist single object steganalysis.With the selected covers,the security of steganography is guaranteed against both single object steganalysis and pooled steganalysis.4.Practical Cover Selection for SteganographyThis dissertation focus on a practical scenario of cover selection for steganography,in which a part of the available images of the sender have been processed to improve visual quality,e.g.,contrast enhancement,image denoising.A cover selection method is proposed to combine the embedding distortion(caused by steganography)and the processing distortion(caused by processing)together to measure the suitability for steganography of the available images.To calculate the processing distortion,a classifier is trained to measure the possibility of the existence of processing.High possibility means high processing distortion.To calculate the embedding distortion,current distortion minimization framework for steganography is employed.Finally,both kinds of distortion are combined to form the total distortion,and the image with the minimal total distortion is selected as cover.This dissertation has an intensive study in steganography in digital images,mainly focusing on distortion function improving for single cover steganography,cover-selection and payload-allocation for multiple cover steganography.Several steganographic methods for steganography in digital images are proposed in this dissertation.These works are helpful to improve the security performance of steganography.