الفهرس 
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Abstract
With the recent acceleration in research into cryptography, the advent in computer and communications technologies, data is now being produced communicated and ultimately consumedindigitalform. Whilethisbroughtwithitawealthofusefulapplications,however, due to the ease of copying and manipulating digital data, developing effective mechanisms for safeguarding of digital media has become an impelling need. Various cryptographic primitiveshaveevolvedwithtimetomeettheevergrowingneedsofdigitalcommunications systems in both military and civilian applications. The primary goal of the current thesis is the study, analysis and development of these cryptographic primitives. The proposed thesis successfully presented several contributions along these directions, especially in the field of public key cryptosystems, steganography and watermarking.
First, a survey on cryptographic standards and algorithms has been presented to compare different cryptosystems. The concept of cryptography is explained as well as its most commonpracticalproblems. Moreover,classificationofcryptographicalgorithmsaccording to key management scheme is provided. A literature review of the most famous protocols together with some tables of comparison is presented. The main goal of this survey is to answer the question ”What are the differences between these cryptographic schemes from a practical viewpoint?” to identify the distinguishing features of each. In doing so, we highlight the important questions to be asked when weighing up the benefits and drawbacks of each scheme.
Second, a new three-party extension of ElGamal encryption scheme is developed together with its security and performance analysis. Moreover, a comparison between our proposed three-party scheme and traditional two-receivers scheme is outlined highlighting its efficiency.
Third, a new image encryption scheme is proposed which blends the ideas from recent researches into a simple, yet efficient image encryption scheme for colored images. It is based on the finite field cosine transform (FFCT) and symmetric-key cryptography. The FFCTisusedtoscrambletheimageyieldinganimagewithauniformhistogram. TheFFCT is chosen as it works with integers and hence avoids numerical inaccuracies inherent in other
transforms. Fractals are used as a source of randomness to generate a strong keystream employed
in symmetric enciphering step. The fractal images are scanned in zig-zag to ensure
decorrelation of adjacent pixels values in order to guarantee a strong key. The performance
of the proposed algorithm is evaluated using standard statistical analysis techniques. Moreover,
resistance to differential attacks measures is investigated. The obtained results show
great potential of the proposed scheme and competitiveness with other schemes in literature.
Additionally, the algorithm lends itself to parallel processing adding to its computational
efficiency.
Fourth, a new approach for secure hiding of textual data (hidden message) in a color
image (cover image) is provided. Use of images as a cover media in steganography is based
on the deficiencies in the human visual system (HVS). The proposed technique employs two
independent chaotic sequences for specifying the locations where the message bits are embedded
in the cover image using an adapted version of the least significant bit (LSB) method.
Message bits are embedded using the 3-3-2 LSB insertion method for the chaotically selected
pixels of the cover image. This technique provides sufficient security as the same sequence of
numbers cannot be generated without knowing the exact key; that is; the initial conditions of
the two chaotic maps used in the index selection process. Moreover, the preliminary results
ensure that eavesdroppers will not have any suspicion that there is a message hidden within
the sent image since the peak signal to noise ratio (PSNR) is high and the mean squared error
(MSE) is low. Furthermore, the length of the secret message is another important component
of the key, which standard steganography detection methods cannot estimate correctly.
Finally, the proposed approach provides better PSNR values and MSE values compared to
other existing techniques as apparent from our experimental results.
Fifth, the proposed hiding approach is extended for secure hiding of an image into a
color image (cover image). Similarly, the results ensure that eavesdroppers cannot detect the
existence of hidden data within the transmitted image since the PSNR is high and the MSE
is low. Furthermore, the size of the secret image cannot be estimate correctly.
Finally, a digital image watermarking scheme is provided based on ElGamal cryptosystem
to hide a grayscale image called ”watermark” into a larger grayscale image called ”cover
image”. In the encryption process, we encrypt the hidden image using ElGamal encryption scheme to obtain the encrypted watermark. In the embedding process, we use two methods
for embedding of the encrypted hidden watermark into the cover image. The first one
works in the spatial domain by embedding the binary values of pixels of the encrypted watermark
into LSBs of cover image pixels values. The second one is a frequency domain based
technique, where the discrete Haar wavelet transform of the encrypted watermark replace
selected coefficients of the transform of the cover image so that hiding the encrypted watermark
in these coefficients does not significantly affect the quality of the cover image. As a
final step, the inverse transform is applied to store the watermarked image as a regular image,
in any standard format. This image is transmitted through channel, and at the receiver’s side,
the embedded watermark is extracted from the watermarked image ensuring the authenticity
of the received image. Theoretical analysis and computer simulation have verified the successfulness
of the proposed image watermarking scheme. The proposed approach provides
competitive results compared to other schemes in literature.