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I willl offer 1 BTC to anybody who can provide me with a way to make the lottery provably fair in issuing tickets and picking out winners. The system described should be able to handle hundreds of millions of tickets and verification for users should be fairly easy and straightforward. You can post your ideas here and I'll go through them and reward the idea that we use.
How about providing blockchain transaction links in the "previous round winners" list for starters?
Not as much verifiable as most people would want but it gives at least some transparency, right?
Not everyone withdraws immediately and ignoring privacy issues posting a blockchain transaction wouldn't really be proof of anything. I could just send bitcoins to my own addresses and claim that those are payments for winners.