It doesnt encrypt a thing, and, in fact, it even writes the actual name of the properties in ascii if used with blueprint variables. Mine is a very simple one with 2 variables, and its something like header, then UE4, then some data, then DWScoreSave_c (name of my class) then BestScore(name of my property) some data, then FloatProperty, then some more data. Its extremelly easy to modify and cause havoc.
I guess you could try modifying the serialization functions so its encrypted, or, easier, add a hash. For example, in my case i have 2 scores, so i could write a 3rd variable that is a hash from those 2, then, when i load, i check that is valid, and if it isnt, i know its been tampered with, but its still a lost cause, if people want, they will probably crack it anyway.