To avoid attracting their attention in the first place. They are not going to bother breaking every 256-bit-encrypted thing they see but as soon as they notice something obviously 512-bit-encrypted they go beat the key out of you.
In cryptanalysis it is normally assumed that the attacker knows the algorithm and encryption parameters aside the key.
I’m not interesting so not an issue for me, but I would also assume that they can break much more than this and wouldn’t actually reveal their limit in such a policy. Kind of like how your boss quadruples your time estimate. If their policy says nothing over 256, then they likely have capability for more.
But I’d expect that usually these pass phrases can be any length as they are all getting beaten out of us when needed.
There are several degrees of interest. Essentially they are interested in everyone trying to hide anything from them and they probably understand people of real danger to them will do their best to look ordinary and mediocre.
> If their policy says nothing over 256, then they likely have capability for more.
Sounds reasonable but breaking 512 obviously is much harder than 256 and will take significant time and resources they don't really want to waste. They will rather force you to reveal the key and only break 512+ if they can't easily reach you physically or identify you in the first place.