How to create a write-protected file from a c# script?

I try to save the Data of the player in some files using System.IO.File.WriteAllText. Is there any way to avoid players from changing their data by hand? Nobody should be able to get as much Gold as he wants just by changing the text in one unhidden, unencrypted file. Has somebody a good way to encrypt a file? Or to create a write-protected file? A password-saved file? I am looking for every possibility. melwei