What are the "natural language input messages" that were used to derive Streebog's constants?
If you reverse the bytes, and you decode in cp1251, you get a series of Cyrillic names.


$C_1$: Гребнев
$C_2$: Сергей Владимирович
$C_3$: Дмух
$C_4$: Андрей Александрович
$C_5$: Дыгин
$C_6$: Денис Михайлович
$C_7$: Матюхин
$C_8$: Дмитрий Викторович
$C_9$: Рудской
$C_{10}$: Владимир Игоревич
$C_{11}$: Шишкин
$C_{12}$: Василий Алексеевич


I leave it as an exercise for the reader to find who all these people might be.  Hint: Рудской transliterates to Rudskoy.