Cryptanalysis of GOST2

GOST 28147 is a 256-bit key 64-bit block cipher developed by the USSR, later adopted by the Russian government as a national standard. In 2010, GOST was suggested to be included in ISO-18033, but was rejected due to weaknesses found in its key schedule. In 2015, a new version of GOST was suggested by Russia’s standardization body (TC 26), with the purpose of mitigating such attacks. In this paper, we show that similar weaknesses exist in the new version as well. More specifically, we present a fixed-point attack on the full cipher with time complexity of 2 encryptions. We also present a reflection attack with time complexity of 2 for a key that is chosen from a class of 2 weak keys. Finally, we discuss an impossible reflection attack and several possible related-key attacks.