Cryptography and Steganography in Digital Images

Master's Thesis from the year 2009 in the subject Computer Science - IT-Security, grade: 19.61 / 20, , language: English, abstract: Cryptography is a technology intended to provide protection to digital data being transmitted over communications systems. It provides mathematical methods for converting data into an unreadable form. In the other hand, the Steganography is a technique by which the secret digital data is sent from one side to another by embedding it into an honest-looking digital cover, so the existence of hidden data is concealed. The data to be hidden takes the form of any bit stream while the digital cover can be a digital image, audio, or video file.

Cryptography and Steganography can work together where combining them provides two-layers protection; The former encrypts the secret message, so it becomes unreadable; the latter hides the encrypted message into an honest-looking digital cover, so it can not be seen. In this thesis, we briefly review the principles of cryptography and its algorithms, while we achieve a detailed study in the field of JPEG-based steganography since JPEG format is the most frequently used through the internet.

In Chapter 2, we present the structure of JPEG format and the related subjects; DCT transformation, and entropy coding. In Chapter 3, we present some well known DCT-based methods: F3, F5, and Mod4 and show their properties and features.

Chapter 4 presents a comparison study between DCT-based steganographic methods F5 and Mod4, the study was completed using Matlab software written for F5 and Mod4 algorithms. In this study, three steganographic requirements are considered; insertion capacity, image quality, and statistical properties. The study shows that F5_111 has big insertion capacity but it could be detected easily while Mod4 has small insertion capacity but it can not be detected easily. The study shows that F5_ME takes a middle position between the considered algorithms. A magic triangle model is presented in order to compare visually between the considered algorithms.

Chapter 5 discusses the shrinkage phenomenon occurring during the embedding process of F5; it presents all the effects of shrinkage in the F5 algorithm. In order to reduce the shrinkage rate, a new method F5_MOS is proposed, F5_MOS controls the shrinkage rate by building a Map Of Shrinkage. A comparison study is done between the F5_MOS and the traditional F5_ME considering the main steganographic requirements. The comparison shows how F5_MOS provides more insertion capacity and how it makes the hidden message more undetectable.