There is always a way around encryption of any kind.. its a nice idea but as LordShryku they will need a decryptor on the server which means a download and again hackable source.
It also depends on the security of their server.
Lots of things to consider, and not sure what people around here think but i would go as far as impossiable. That is to make it completly unreadable..
As I said and many here before have said there is always a way.
But good luck