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Impossible Differential Cryptanalysis of Lightweight Blockciphers TEA and XTEA

Hajari, Masroor | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48611 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Aref, Mohammad Reza; Salmasizadeh, Mohmoud
  7. Abstract:
  8. The lightweight blockciphers and lightweight authenticated encryptions are special kind of the symmetric schemes designed to use in cases that resources such as power, area, etc. are limited. Sensor Networks and RF-Tags are two good example of lightweight schemes application. This limitations force designers to use simpler functions and operators in comparison to conventional schemes; therefore, providing the security in this case is so important. A novel approach for evaluating security of an algorithm is to apply different kinds of attack. Impossible differential cryptanalysis is one of the shortcut cryptanalysis, using the impossible differential characteristic. Impossible differential characteristic is a differential characteristic based on logical contradiction; thus, each corresponding key that satisfes conditions of impossible differential characteristic is a wrong key and must be eleminated from the candidate key set. Finally, the attacker search the keys remained in the key set in order to fnd the correct key.TEA and XTEA are two lightweight Feistel block cipher algorithms that each of them have a block size of 64 bits and a key size of 128 bits. These two ciphers have ARX structure, i.e. their round functions consist of modular addition, shift and XOR to be exact. Since each operation of TEA and XTEA can be done in a bitwise fashion,we are able to use early abort technique that helps us to remove inappropriate pairs as soon as possible, hence decreasing time complexity. Moreover, we use the online key search method. This method let us save the appropriate pairs instead of guessed round keys. This feature can reduce the amount of capacity, necessary in cryptanalysis of two algorithms. In this thesis, we enhance the previous attacks by appling impossible differential cryptanalysis to TEA and XTEA. Then we demonstrate that the cryptanalysis issued by Chen et al. utilizes a wrong approach. In addition, we introduce the two new cryptanalysis mounted on 17-round and 23-round of XTEA. Furthermore, merging the two new 15-round impossible differential characteristics, which demonstrated in this thesis, resulting in the frst known impossible differential cryptanalysis mounted on 19-round TEA and 25 rounds of XTEA
  9. Keywords:
  10. Impossible Differential Cryptanalysis ; Lightweight Block Cipher ; Authenticated Encryption ; TEA Block Cipher ; Block Cipher

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