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[So what would be needed to remove the possiblity for the site operator to defraud the investors, would be an verifiable external source of entropy over which the site operator has no control and that all participants can verify independently.
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The way other gambling houses do this is have a random number generator that can be observed by the investor and gambler simultaneously. Like a roulette wheel, roll of actual dice, lottery tumbler or even a horse race. If both parties see the number generation simultaneously and agree on it's fairness then neither can know the result before the other. So the problem with just-dice's number generator is that the house and player can't observe the generation simultaneously. The house controlling the generator mechanics always has a way to know the outcome before the player.
I can't think of a simultaneously observable generator that can do 20 bets a second. The lottery tumbler on a live video feed is the best I can think of.
Maybe if the actual time can be used to seed or hashed with the number generator? Can everyone agree on what time something happens? Does this help? Not if it creates a predictable generator. Just thinking out loud here. I don't really know.