Any good method to encrypt JIC file?

Reading back the contents of an EPCS chip is very easy. It is a standard serial SPI chip. But if you use the Cyclone III LS, the data on the EPCS will be encrypted, with the key stored in the FPGA itself. So even if someone reads back the contents of the chip, it will be impossible to get back the original contents or to copy it to another board without the FPGA's key.