Is there a way to encrypt design under Quartus?

If your intent is just for your customer to *try* your IP, then you can just provide them the bitstream file (.sof), and you don't even have to give them your design files (source HDL, and even QXP partition).

Your customer can just work with your .sof file if he has the right board to play with.

I'm not sure what you really want to achieve here. Do you want your customer (or who you're dealing with) to compile your design, or just test / try your solution on a board? If compile is necessary, then QXP or full HDL sources need to be given, otherwise, just the bitstream file will do.