I would do the drawing as follows:
- Go to random.org and generate the random number(s) between 0 and n based on a "persistent identifier". A good identifier to use would be a newly generated bitcoin address that has not been used yet.
- Post the SHA-256 hash of the identifier on the forum to commit to the number(s)
- Publish the guesses. This way, people can see if all guesses have been exhausted. This prevents me from claiming no-one won.
- Announce the winner and publish the identifier used to generate the number(s), so everyone can verify the winning number(s).
This is a very good solution. I was looking around on random.org, but I hadn't thought it all the way through. Would you be interested in buying into a lottery like this? It's interesting, because it doesn't reward buying a bunch of guesses at once (each one gives diminishing returns, because you're putting more money into the pot) and the longer it goes on (maybe capped to a week, and whoever's closest after a week wins) the more profitable it would be, because there'd be less choices, thus more chances for winning. It's almost like a Bingo lottery. =P