I have a question, how exactly does provably fair work? I've wondered for a while, and how can I verify if it is provably fair? It would be awesome if there was some sort of plugin for Chrome/Firefox that would automatically check if it was provably fair.
It works similarly on all provably fair sites, pretty much.
1. the server picks a random "server seed"
2. the server shows the user a hash of the server seed
3. the player picks their own seed, the "client seed"
4. the server uses the server seed and the client seed to generate the outcome of the game
5. the server publishes the server seed
Notice that it's important that 1. comes before 3. The player picks last, so there's no way the site can influence the rolls.
Once all five steps are complete, the player can check that:
a) the server seed published in step 5 does in fact hash to the string that was shown in step 2, and
b) the client seed (step 3) and server seed (step 5) do in fact generate the shuffle, roll, spin, or whatever that happened in the game (step 4)
That's all. Some sites (satoshidice, coinroll) change the server seed every 24 hours. Some sites (primedice, bitzino) change it every roll. Some sites (just-dice) change it on demand. I prefer the last way - 24 hours is a long time to wait to verify a suspicious roll, but changing the seed every roll makes for a lot of work for the player, making a note of each hash, verifying each seed, etc.
For Just-Dice, there's a user-contributed script at
http://jsfiddle.net/Zq8tL/3/ which lets you tell it the client seed and server seed, and outputs all the rolls those seeds generate. So you can check it against the rolls you actually saw and make sure there's nothing fishy going on.
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Edit on 28 Nov 2013: at the end of September 2013, from bet 145 million onwards, the algorithm used to generate rolls
was changed and so the above-linked jsfiddle script no longer works.
There's a
new verifier linked from the bottom of the 'Fair?' tab which is able to apply both the old and new algorithms, and defaults to the new one.
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