[lamacase]

But CRC checks wouldn't prevent things like revealing units through fog of war right? The presentation of the game state couldn't be CRCd because each player has a different view of it. And the cheat client doesn't have to modify the actual game state to get that information out.

[alternatetwo]

The game actually tracks this too, what is visible for each player, which gets slightly complicated with diplomacy changing. Of course this doesn't prevent you from just patching other parts handling this, but you cannot just simply modify this value, it will desync as well.