Weak keys of the full MISTY1 block cipher for related-key amplified boomerang cryptanalysis

The MISTY1 block cipher has a 64-bit block size, a 128-bit master key, and a total of 8 rounds. It is an ISO international standard, a Japanese CRYPTREC-recommended e-government cipher, and a European NESSIE selected cipher. In this study, the authors show another cryptographic weakness of the full MISTY1 cipher: they describe four classes of 2⁹⁰ weak keys of the full MISTY1 cipher for a related-key amplified boomerang attack that has a data complexity of 2^60.5 chosen plaintexts and a time complexity of 2^87.33 encryptions under each class of weak keys. The result shows that the MISTY1 cipher can be distinguishable from an ideal cipher in terms of related-key amplified boomerang cryptanalysis, and users should be very careful when using MISTY1 for a full security in relevant application situations.