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    Blok zinciri platformları, fikir birliği mekanizmaları ve ağın güvenlik analizi
    (Haliç Üniversitesi, 2022) Aslan, Mimar; Kasapbasi, Mustafa Cem
    Finans, sağlık, sosyal medya vb ortamlardaki insanların ihtiyacı olan güven problemine blok zinciri teknolojisi, şifreli algoritmalar ile çözümler sunmaktadır. Güven problemini çözen ve verileri dağıtık olarak kayıt altına alan ve her şeyi şeffaf olarak bizlere sunan blok zinciri bir devrim niteliğindedir. Blok zinciri, akıllı sözleşmeler sayesinde kurumsal projelerde de kullanılmaktadır. Kurumların arasında yeni nesil bir ağ olarak da adlandırılan blok zinciri ile birçok şeyin değişmesi beklenmektedir. İnternetin, mobil cihazların ve sensörlerin yaygınlaşmasıyla birlikte güven problemi her geçen gün daha da önem kazanmaktadır. Farklı amaçlara hizmet eden Ethereum, Cardano, EOS, Cosmos, Hyperledger gibi blok zincir platformları bulunmaktadır. Altyapılarında Proof of Work (PoW), Proof of Stake (PoS), Delegated Proof of Stake (DPoS) ve Directed Acyclic Graph (DAG) gibi farklı fikir birliği mekanizmalarını kullanmaktadırlar. Bu çalışmada blok zinciri platformları, altyapılarında kullandıkları mutabakat mekanizmaları ve blok zinciri ağının güvenliği incelenrek araştırılmıştır.
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    Enhanced chaotic key-based algorithm for low-entropy image encryption
    (IEEE Computer Society, 2014) Yavuz, Erdem; Yazici, Rifat; Kasapbasi, Mustafa Cem; Yamac, Ezgi
    As computing power of today's computer enhances rapidly, secure encryption is getting harder and the need for powerful secure encryption methods becomes inevitable. To avoid brute-force attacks it is necessary to have a larger key space, which makes the chaotic systems more attractive. In order to sufficiently apply confusion and diffusion rules for an image data with any entropy structure, a two-dimensional chaotic system, which has high sensitivity to initial states, has been utilized in this study. The fact that adjacent pixels of an image have naturally close values makes it easier for crackers to perform statistical analysis. So, two chaotic functions are used to disrupt correlations among them, one for shuffling pixel positions and one for changing pixels values. Even with low entropy images the proposed algorithm has been proved to be more secure and faster than the previous algorithms. © 2014 IEEE.
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    Enhanced Chaotic Key-Based Algorithm for Low-Entropy Image Encryption
    (IEEE, 2014) Yavuz, Erdem; Yazici, Rifat; Kasapbasi, Mustafa Cem; Yamac, Ezgi
    As computing power of today's computer enhances rapidly, secure encryption is getting harder and the need for powerful secure encryption methods becomes inevitable. To avoid brute-force attacks it is necessary to have a larger key space, which makes the chaotic systems more attractive. In order to sufficiently apply confusion and diffusion rules for an image data with any entropy structure, a two-dimensional chaotic system, which has high sensitivity to initial states, has been utilized in this study. The fact that adjacent pixels of an image have naturally close values makes it easier for crackers to perform statistical analysis. So, two chaotic functions are used to disrupt correlations among them, one for shuffling pixel positions and one for changing pixels values. Even with low entropy images the proposed algorithm has been proved to be more secure and faster than the previous algorithms.
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    A new ımproved lsb chaotic ımage steganography scheme
    (IEEE, 2022) Kasapbasi, Mustafa Cem; Bayam, Idris
    The purpose of the Steganography is to hide and transfer secret or sensitive information/message in a multimedia medium, so that it can not be understood by unintended parties. In the literature there are variety of techniques utilizing spatial domain image steganography but enhanced steganography schemes are still needed for improving quality of stego-image. In the proposed work, a new spatial domain steganography scheme enhanced by chaotic maps is presented, namely Tent map, Logistic map, Bernoulli Map, Quadratic Map, Chebyshev map and Sine Map in different combinations to select the pixel location for hiding data. A compression is carried out on the secret message before hiding in the carrier image in order to improve secret message amount. Assessment of the proposed system is carried out not only with quality of the stego image statistical values like PSNR, MSE and histogram but also in terms of, entropy, correlation for each chaotic map. It is concluded from the statistical test results that the proposed scheme is imperceptible enough for hiding the information in a digital medium. Moreover the proposed system has passed rigorous Stego-Analysis tests with stego analysis tool Virtual Steganographic Laboratory (VSL)