(Igor Sikorsky Kyiv Polytechnic Institute, 2023) Zvychaina, A.; Fesenko, A.
It turned out that in addition to problems with classical asymmetric cryptography in the post-quantum period, there are certain doubts about the strength of symmetric cryptographic schemes. This paper demonstrates that on Type III Generalized Feistel Scheme (GFS), by selectively fixing specific parts of the plaintext at the input to the GFS, it is possible to reduce the problem of distinguishing between random text and encrypted output of the same GFS to the Simon problem through different approaches. Our method enables the cracking of the cipher up to d rounds in polynomial time, while a more sophisticated approach based on different formulas from other paths of the cipher can crack d + 1 rounds with the same time complexity in quantum adversary model. These distinct approaches yield varying results in terms of scheme security, indicating the potential to break more rounds in the GFS using the same methodology.