Thesis Type: Postgraduate
Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Graduate School of Informatics, Information Systems, Turkey
Approval Date: 2017
Student: ASUMAN ŞENOL
Consultant: AYSU BETİN CANAbstract:
Differential attacks aim to capture the round keys by examining the changes in the output when a small change is applied to the input. This method is based on examining the differential behavior of the cryptosystem and guessing the affected round keys by using candidate plaintext and ciphertext pairs. It was shown that it may not be possible for the attacker to fully uncover the guessed keys. This situation occurs when the cipher contains S-boxes after the key addition layer and the guessed keys have a specific difference for a fixed S-box output difference for some S-boxes. Such an S-box property is called a differential factor. Because of the uncovered keys which is caused by not taking differential factors into account, attacks in the literature obtained by theoretical methods may not work correctly in practice. In addition to that, more powerful differential attacks can be proposed with undisturbed bits because these bits provide discovering longer differential characteristics. As these attacks are corrected by considering differential factors and undisturbed bits, the claimed time complexity may increase or decrease. Rectangle and Present are two lightweight block ciphers with SPN structure and their S-boxes have differential factors and undisturbed bits. In this work, we corrected previously published differential attacks on Rectangle and on Present by the help of undisturbed bits and we showed that these attacks can actually be performed with time complexities reduced with the help of differential factors and undisturbed bits .