DIFFERENTIAL CRYPTANALYSIS ON LBLOCK USING DIFFERENTIAL FACTORS

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Tezin Türü: Yüksek Lisans

Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Fen Edebiyat Fakültesi, Matematik Bölümü, Türkiye

Tezin Onay Tarihi: 2018

Öğrenci: MERVE ÖĞÜNÇ

Eş Danışman: CİHANGİR TEZCAN

Danışman: ALİ DOĞANAKSOY

Özet:

Cryptography had actually a long history and comes to today by evolving day by day. Now, it is a huge area in terms of the applications in industry and research topics in academia. Even if we do not realize, the cryptographic tools are placed in every single day of our life. To protect the information, the cryptographic algorithm is used in several areas from the basic website to smart devices. One of the classes of cryptographic algorithm is symmetric-key algorithms which cover block ciphers and stream ciphers. To evaluate the security of block ciphers, several cryptanalytic methods are used as a tool in cryptography. One of the most important methods is differential cryptanalysis. Since it is commonly used, cipher designers specify the cipher principles to be secure against differential attack. In differential cryptanalysis, attacker observes that the difference between chosen plaintexts how affects the difference between corresponding ciphertexts. After finding a relation between plaintext and ciphertext, an attacker tries to get round keys. With the recently introduced S-box property called Differential Factors, all of the attacked key bits may not be determined if the S-box has a differential factor property and that S-box is activated in the distinguisher. With advances in technology, the usage of embedded systems has increased and the needs for new cryptographic instruments has emerged. Therefore, the subclasses of cryptography become diversified. One of the diversification is lightweight cryptography. Lightweight cryptography is based on optimizing the trade-off between security, cost,and performance. With increasing use of low resource devices such as RFID tags and sensor networking in different areas,the needs for lightweight cryptographic modules have started to increase. For this reason, lightweight cryptography has become prominent for the last few years. To fulfill the need, several lightweight block ciphers have been designed such as PRESENT, SEA, LED. In this work, we briefly present some lightweight block ciphers, their cryptanalysis and corrected cryptanalysis via differential factors. LBLOCK, as one of these lightweight block ciphers, is a 32-round block cipher proposed at Applied Cryptography and Network Security Conference 2011 by Wenling Wu and Lei Zhang. In this thesis, we study on the lightweight block cipher LBLOCK and observe the differential cryptanalysis to LBLOCK.Since the attackers do not consider the differential factors while performing the attack,the time complexity needs a correction. We correct the time complexity of the attack.