Research On Multi-bit Constraints Of ARX Structure And Its Differential Property

ARX structure is a kind of cipher unit involving three kinds of operation: Modular addition, Rotation and XOR. Because these operations can interact with each other in complex ways, and the ARX ciphers contain a lot of iteration rounds, it is difficult to analyze the security of ARX ciphers against differential attack. Up to now, the results of differential research in ARX structure ciphers are less. So it is a great valuable and theoretical significant subject to study the security of ARX designs against differential analysis. In this thesis, in order to study the differential properties of ARX structure, two aspects were studied. The one is multi-bit constraints, and the other one is searching for differential characteristics. The main research work is as follows:1. Based on the study of multi-bit constraints, we analyzed the incompatible situations of modular operation. We used the multi-bit constraints to check the compatibility of known differential characteristics presented in Skein, and found some of them were incompatible for Skein. We also found the incompatible situations of the units containing modular operation and rotation operation.2. We introduced an algorithm to search for differential characteristics for ARX ciphers, and used our searching algorithm to find differential characteristics of Chacha which is ARX structure. We got a 3-round differential characteristic and its weight is 13. Preprocessing analysis of ARX ciphers and accuracy test of input and output difference were integrated into the searching algorithm. Compared with the existing searching algorithm, the searching algorithm has greater pertinence and effectiveness.