Cryptanalysis Of Block Ciphers

An investigation of the cryptanalysis techniques of the modern block cipher is taken in this thesis. We have focused on applying the traditional cryptanalysis techniques on the AES (Advanced Encryption Standard of USA) and SMS4 (block cipher for WAPI). Our contributions are summarized as follows.1. All published papers on the Impossible Differential Attack on AES are discussed. Some similarities among them are summarized and a general impossible differential attack on 7-round AES with varied key length is presentd. Such attack takes the number of all-zero columns of the 7th and the 6th round as parameters(α,β). And a trade-off relation between the number of plaintexts and times of encryptions in the process of the attack is derived, which makes only some values of(α,β)allowed in the attack for different key length.2. In order to evaluate the security against the differential cryptanalysis of SMS4, we design a method to calculate the lower bounds on the number of active S-Boxes for all kinds of differential characteristics (or differential patterns). Firstly, we divide the pattern into ten kinds of sections, the lower bounds on the active S-box of which are calculated in detail. Then the lower bounds on the active S-box in all combinations of the ten kinds of sections are derived. Finally, we show that there is no differential attack against more than 31 rounds SMS4 based on certain differential characteristic.3. Discuss the development from the Boomerang attack to the Rectangle attack. The necessary condition of the existence of Amplified Boomerang distinguisher on the block cipher SMS4 is presented. And it is analyzed how to choose the the plaintext quartets. Then, an example of a 14-round rectangle distinguisher is discussed and proved to be inexistence. Finally, the necessary condition of the existence of Amplified Boomerang distinguisher on the block cipher AES is also discussed.