LOL. if you want to make it so cheating isn't possible

you'd have to have email between one program for white, one for black, corresponding with still another program as referee.
at that point the software becomes a redundancy - if you have a 3rd person you don't need the software. now a kriegspiel SERVER...
any basic encoding to prevent accidental reading like base64 should be more than sufficient. i think you're wasting time on the online bit but good luck.