just reserected an old design which encrypts serial data stream ie rs232 with rc4 and transmits it as manchester coded data,its a two chip solution,both being cheap xilinx cplds,one contains the serial encryption unit and transmits the manchester code,the other has a manchester decoder and a decryption unit.In my original design you just squirted serial data in one end and the same data came out the other and is very difficult to intercept in between.I am sure its the easiset way rather than messing with block ciphers.