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Through the spread of Bitcoin for the purposes of online gambling, the concept of "provably fair" games came more into the limelight. Which of the Bitcoin-powered gambling websites are provably fair?
For clarity's sake, a provably fair game is a game that uses cryptographic algorithms to ensure their players that the game host or other players did not temper with the outcome of the game after it has begun.
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BitLotto was likely the first blockchain-based "provably fair". It uses the transaction ID as the "ticket", and the results of a large government-run lottery to determine the winning numbers.
SatoshiDICE became a blockbuster (pun intended) method for online gambling. It uses the transaction ID to determine the "lucky number" (between 0 and 65,535). The secret used for determining that lucky number is revealed after the end of the day.
bitZino gets credit for coining the term "provably fair", and first launched an online blackjack game then followed it up first with video poker and then roulette as well. The game allows the user to change the client seed used for each play.
BitJack21 was possibly the first. The game gives the ability for the player to change the seed used for each hand.
BitDice.com, SatoshiRoulette.com and others claim to be provably fair, I believe. But I haven't gone through the verification steps myself to confirm.
BitLotto and SatoshiDICE wagers can be verified by anyone at any time after the results are published in BitLotto's instance, and after each day's secret is published for SatsoshiDICE's instance. Proving that bitZino always computed the results correctly requires logging of the hash, the client seed, and also the way the hand was played -- so it is much more effort to verify unless there is automation but no automated solutions for verifying all wagers on bitZino have been made available.
Here's an article on what provably fair means:
Here is one explaining how it works:
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Additional sites in which claim to be provably fair but I've not gone to verify:
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I run a provably fair casino - BitSaloon. I'd love feedback on how we can improve.
http://www.bitsaloon.com/about/fair
Random numbers are generated with sha256(salt + tx_hash) as a seed to Alea (an open source random number generator). If your generated number is less than or equal to the win percent, you get paid. All of this is verifiable by a user.
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Liberty Bets is also provably fair.
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The bitcoin casino site SatoshiBet is 100% Provably Fair. After each deal the player can manually verify the legitimacy of the shuffle.
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Bitoomba Casino supports provably fair or fairness verifications the site protected with SSL technology.
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Also, DoubleDownBTC, a Bitcoin Blackjack game, has a good explanation about provably fair gaming at https://doubledownbtc.com/fair
They implement one of the more popular Provably Fair models using a combined Server and Client seed to seed a shuffle on a deck of cards. They return the hash of the server seed before the player creates their seed so that the game can be verified afterwards. They also have a popup "Verifier" which will do this live.
1Hi Ryan! You mention this site in several answers. Is it your site? If it is you should probably disclose it. As you can see all your answers tend to get downvoted. – D.H. - bitcoin.se – 2013-09-18T18:36:57.060
1I just rejected a proposed anonymous edit for "bit millions". They say provably fair, but the random seed comes from a twitter account they may control. I couldn't easily prove otherwise. – goodguys_activate – 2013-03-15T17:02:41.787
If I missed any, I'll add them, just let me know. – Stephen Gornick – 2012-09-29T08:44:14.803