Sorry for getting off-topic, but RPS is very easy to make provably fair. In fact I do it all the time. Here's a good example of me playing in a provably fair way (for both of us) just using this forum: https://bitcointalk.org/index.php?topic=1588904.0 and includes a lot of explanation of how it works

Disclaimer: I'm down like 15 BTC net Fuck RPS