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Maybe try incorporating with the block hashes. Give a predetermined time the results will be pulled and a upfront predetermined hash everyone can see. 24 hours later or whenever the drawing is grab the latest hash from the most recent block. Now you just need to convert those hashes into integers and then do some math that will spit out a # between 1 and the number of participants that can be assigned to the users who deposit/play.
The server end is running on bitcoinj. For this first winning the code was just using Java's math.random to generate the seed. Next week and beyond it'll be using something incorporating the blockchain. It looks like I can use the blockchain as a bitcoinj object and hash it, then incriment up by a hundred every iteration as the seed for each number, so we can display proof of results.
Edit, just saw that I won the first one (I threw in $3 of the $8 last week), so I'm a bit disappointed. I just submitted the whopping $8 back into the pool haha. Hopefully somebody else gets it this time. It currently stores the previous winners and stuff on the site but I haven't made a way to view that info yet, so I'll work on that soon.
Edit 2, I've started work on the viewing of last week's winnings (as you can see on the site on the bottom). I found a way to get it to display everything I want. So next week, it'll display the winning time's current Block and hash of the block - with the code to run to prove that the winner was chosen at random. You can see some of that info now, but since it wasn't all available for last week it just shows as blank.