A Single-Key Attack on the Full GOST Block Cipher

The GOST block cipher is the Russian encryption standard published in 1989. In spite of considerable cryptanalytic efforts over the past 20 years, a key recovery attack on the full GOST block cipher without any key conditions (e.g., weak keys and related keys) has not been published yet. In this paper, we show a first single-key attack, which works for all key classes, on the full GOST block cipher. To construct the attack, we develop a new attack framework called Reflection-Meet-in-the- Middle Attack. This approach combines techniques of the reflection attack and the meet-in-the-middle attack. We apply it to the GOST block cipher with further novel techniques which are the effective MITM techniques using equivalent keys on short rounds. As a result, a key can be recovered with 2(225) computations and 2(32) known plaintexts.