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 one of the technological means that provides protection to digital data being transmitted on communications systems. It provides methods of converting data into an unreadable form, while the Steganography is a technique by which the secret digital data is sent from one place to other by hiding it within a digital cover so the existence of secret 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.

Combining Cryptography and Steganography provides two layers protection, the former encrypts the secret message so it becomes unreadable; the latter hides the encrypted message into a digital cover so it cannot be seen. In this thesis, we briefly review the principles of cryptography and its algorithms while doing deep work in JPEG-based steganography.

Since JPEG format is the most frequently used through the internet, it is desirable to study some Steganographic techniques based on JPEG format. Chapter 2 presents the JPEG structure and the related subjects; DCT transformation, and entropy coding. Chapter 3 presents some important DCT-based methods like F3, F5, and Mod4 and shows 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.