They use a hash function
A hash function is a way to confirm that two parties both know the same thing without sharing what that thing actually is.
For example, if you are creating a website and need to store usernames and passwords, it is best not to store the password in the database (because the database could become compromised or accessed by an untrustworthy person) Instead you store a hash of the password. A hash is computed by using the values of each letter in the string (e.g., X is 82, Y is 83, etc.) and their positions, which are subject to a mathematical function to create a hash.
So if the SHA256 of my password is 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73 043362938b9824, then that is stored in the database, and when I try to login the password I enter (in this case 'hello') is hashed using the SHA256 algorithm - if you enter hello here:
http://dev.zer0day.com/PHP%20Hasher.php
it will always give that same value. Another characteristic is that a hash function should give a completely different hash for "hello" compared with say "hellp", which makes it hard to crack subsections.
There is a video explaining here:
http://www.betvoyager.com/demo/roulette.htm
They are not hashing the next 20 numbers (which would be vulnerable because after you had say 19 numbers, there would only be 37 possibilities for the hash), but only the next number. The are not hashing just the number, because that would leave only 37 possible hashes, but the number together with a randomised code word.
After the spin occurs they reveal the code word, which, when hashed with the result of the spin, will give the pre-shared SHA256.
Because changing one character of the code word would result in a completely different hash, it is impossible for them to change the result of the next spin according to your bet.
This doesn't prove that their RNG is perfect or that the wheel is unbiased, but it does mean that they cannot cheat you.
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