Security Analysis Of Several Block Cipher Models

Differential cryptanalysis and linear cryptanalysis are powerful attacks on block ciphers.Designers have tried to evaluate the security of any proposed ciphers against differential andlinear cryptanalysis. Based on these facts, detailed researches are done about the practicalsecurity of several generalized Feistel models in this thesis. The main works and creations are asthe following:1.The practical security of Nyberg-type generalized Feistel model is researched in detail.For2m-block Nyberg-type generalized Feistel model, the lower bounds of the number of activeround functions for (m+l)-round and2m-round are given by analysing the structure ofdifferential characteristic and linear approximations. For four-block Nyberg-type generalizedFeistel model and its deformation structure, the lower bounds of the number of active roundfunctions for arbitrary round differential characteristics and linear approximations are givenwhen round fiindtions are all bijective, and it is proved that the lower bounds can not beimproved. Furthermore, it is pointed that the deformation structure can provide more activeround functions than four-block Nyberg-type generalized Feistel model.2.The practical security of CLEFIA-type generalized Feistel model is researched in detail.For2d-block CLEFIA-type generalized Feistel model，the lower bounds of the number of activeround functions for6-round are given when round fundtions are all bijective by analysing thestructure of differential characteristic and linear approximations. For four-block CLEFIA-typegeneralized Feistel model，the lower bounds of the number of active round functions for arbitraryround differential characteristics and linear approximations are given when round fundtions areall bijective by using the dual relationship between the deformation structure of four-blockNyberg-type generalized Feistel model and the four-block CLEFIA-type generalized Feistelmodel, and it is also pointed that the lower bounds can not be improved. When SP structure isused in the round function, the lower bounds of the number of active S-boxes for6-rounddifferential characteristics is given.3.Four-block CLEFIA-like generalized Feistel models based on two shift transformationsare proposed, and the practical security of this model is researched in detail. The lower bounds ofthe number of active round functions are given for arbitrary round when round functions are allbijective by analysing the structure of differential characteristic and linear approximations. Andit is also pointed that the lower bounds can not be improved.