On the Security of Qalqan Cipher Against Differential Cryptanalysis

In 2021, the first version of block cipher Qalqan was presented. It is positioned as a candidate to the future national encryption standard of the Republic of Kazakhstan. This cipher features the usage of addition by different modules for mixing the round keys and for linear layer. In this work, we consider some cryptographic properties of Qalqan, related with the security against differential and linear cryptanalysis. We present variations of cipher’s S-box with better cryptographic properties. We prove that branch number of Qalqan’s linear layer is equal to 3, and the layer itself has a significant amount of fixed points. Also, we build a set of multi-round differential characteristics with high probabilities for the modified version of the Qalqan cipher, which uses only addition modulo 256. With these results, we can argue that the declared security of Qalqan against differential and linear cryptanalysis should be reconsidered.