Strength of MISTY1 without FL Function for Higher Order Differential Attack

摘要

The encryption algorithm MISTY is a "provably secure" one against Linear and Differential cryptanalysis. Since the designer showed 3 round MISTY1 without FL function is provable secure, we omit FL to estimate the strength for Higher Order Differential Attack. This attack is a chosen plain text attack and uses the value of higher order differential of output to derive an attacking equation for sub-keys. The value depends on the degree of output and the degree depends on the choice of plain texts. We show that the effective chosen plain text and 5 round MISTY1 without FL is attackable using 11 different 7th order differentials. And we show the attacks to remaining sub-keys by the determined sub-keys and intermediate values. Intermediate value is a constant in the process of encryption. As the result, we can determine all sub-key in 5 round MISTY1 without FL.