Bitcoin 50-50 Style Draw

[jeffayle]

http://bitdraw.nfshost.com/

All information needed is on the site, but a quick summary:

- You buy tickets at 1BTC each
- At the end of each month, a winner is chosen manually (ie. it is not based Bitcoin blocks like some other contests)
- The winner will receive 80% of all ticket sales, 10% will be given to a charity that will be chosen at the end of the month and 10% will be given to the maintainer (me).
- Although the charity will not be known at the beginning of the month, I will try my best to choose either charities whose actions are in line with the Bitcoin mentality or charities who support a cause that most people should support.
- This means that it won't be something like Right to Life, political parties, or anything of that sort.
- Charities supporting getting help to those who need it should be safe, assuming they are not strongly allied with a religious organization.

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[Local]

That's all pretty bad imo.

Winner drawn manually? Unannounced charities? And in return for this people pay 10% vig?

I'll pass.

[jeffayle]

Hmm, you're right. Not announcing the charities is a bit, off. I'll post that at the beginning of the month (I'll edit the site later to reflect that, but I'll tell you now that it will be torservers.net). The reason winners are drawn manually is because I wanted it to have a solid ending time, instead of when block xx appears. I could do something like the last block to appear, but I could still be accused of cheating because the block was not chosen beforehand. The other problem is that I cannot think of any way to use that information to choose a random number in a range of possibly anything, the other sites I've seen that use this have a specific range they use, which it can use to get an evenly distributed random number. The bitcoin lottery solved the problem by requiring you to pick a number for your ticket, like a normal lottery. I didn't want to do this, mainly since it's already been done for bitcoin. But if anyone has any suggestions on how this could be more fair, I'll listen.

And I don't think 10% is too much, but if a few people have objections, I'd be willing to reduce it.

[FreeMoney]

Cool, I like torservers. I don't think 10% is too bad for something well run and transparent. You don't need a winner at an exact minute, right? Why not use the hash of the 10th or 100th block after some time you define as the end of entries?

[jeffayle]

My math might not be correct here, so if someone could correct me if I'm wrong, I'll gladly use hashes, but my problem is that they cannot be used to create an evenly distributed number within an arbitrary range.

If we assume that the hash as a whole is fairly random (which I don't think there's any evidence to doubt this, and there's probably evidence to support it), we can't use that directly to give us a random number in an arbitrary range, the way you'd do that is to take the hash modulo the number of tickets you have sold, the lower numbers would be more likely to come up (there would be a cut off point where numbers below it would be more likely to come up than the numbers above it). For example, if your hash is in the range [0,10) (I'll use a low number here for space constraints, the same thing would happen with any number), and you have six tickets sold. If you take the hash modulo the number of tickets you have, you would end up with {0, 1, 2, 3, 4, 5, 0, 1, 2, 3}. You can see that [0,3] is more likely than four or five if you randomly picked one of those (the one you pick would be equal to your hash). This does work though if the range of your hash is a multiple of the number of tickets you've sold.

While writing this though, I realized that I could probably use the hash value as input to a random number generator in the range of the number of tickets. I don't know too much about random number generators, but if someone could give me some more information on what algorithm I'd want to use for that purpose.

And then while writing that last paragraph, I came up with another solution; if I took the hash value of the first n blocks after the end of the month, where n is the smallest prime factor of the number of tickets, and added them together, I'd get an evenly distributed number in a range that is a multiple of the number of tickets sold. Is that mathematically sound?

If I did anything like that, I'd make sure to keep a list of all tickets currently sold at all times, so that anyone can verify I'm not inserting my own tickets at the end.

[Anonymous]

http://www.random.org/   ?

[jeffayle]

http://www.random.org/   ?

That's my current plan, but there's no way to verify it. If I base the winner in a predictable way off the blocks that show up later, anyone can verify that it's fair.

[fornit]

very easy way to solve your problem with the modulo is to use it from both sides. you take
hash value mod participants
when hash value/participants  is even and
participants - hash value mod participants
when hash value/participants is not even

btw the advantage of having a lower number should be be very small anyway. just use it as an incentive for people to register early 
make your fee 5% and let partipants vote the charity and i am in.

[jeffayle]

Yeah, I guess you're right about the incentive to get in early. And, the more tickets that are sold, the smaller the chance difference will be. I'll go with that then.

Also, as for voting on the charity, how's this: when you buy a ticket, you a url to a web page page of a charity that will accept donations with bitcoin. The charity chosen would be the one on the winning ticket? Or would it be better to have a poll to vote? I'll do one of these starting next month, I don't want to change it now that I've already said where the money is going.

Would you prefer (and this goes to anyone) that I increase the amount going to charity or to the user, when I lower my cut?

[jeffayle]

Okay, a winner was chosen and they were sent 5.40BTC.

If I were to make the proposed changes (using the bitcoin block to determine winner, winner chooses charity), would anyone be interested? I just set this up to play with bitcoin, so I won't set it up to go again unless there's some interest.