Related mailing list post: https://groups.google.com/a/bittorrent.com/forum/#!msg/bt-developers/jgwSecIxVcM/itaChdLXLU4J
Edit: It's not really a whitepaper, and it's not really clear what exactly is going on. I'm not terribly impressed. The "diagram of the encryption model" on their website doesn't elucidate anything either.