Question

Given the extremely high requirements for unpredictability to prevent casinos from going bankrupt, what random number generation algorithm and seeding scheme is typically used in devices like slot machines, video poker machines, etc.?

EDIT: Related questions:

• Do stateless random number generators exist?
• True random number generator
• Alternative Entropy Sources

Answer 1

For a casino gaming applications, I think the seeding of the algorithm is the most important part to make sure all games ‘booted’ up don’t run through the same sequence or some small set of predictable sequences. That is, the source of entropy leading to the seed for the starting position is the critical thing. Beyond that, any good quality random number generator where each bit position as has a ~50/50 probability of being 1/0 and the period is relatively long would be sufficient. For example, something like the Mersenne twister PRNG has such properties.

Using cryptographically secure random generators only becomes important when the actual output of the random generator can be viewed directly. For example, if you were monitoring each number actually generated by the number generator – after viewing many numbers in the sequence – with a non-cryptographic generator information about that sequence can lead to establishing information about all the internal state of the generator. At this point, if you know what the algorithm looks like, you would be able to predict future numbers and that would be bad. The cryptographic generator prevents that reverse engineering back to the internal state so that predicting future numbers becomes ‘impossible’.

However, in the case of a casino game, you would (or should) have no visibility to the actual numbers being generated under the hood. Each time a random number is generated – say a 32-bit number – that number will be used then, for example, mod 52 for a deck shuffling algorithm….no where in that process do you have any idea what numbers were being generated by the algorithm to shuffle that deck. That is, most of the bits of ‘randomness’ is just being thrown out and even the ones being used you have no visibility to. Therefore, no way to reverse engineer the state.

Getting back to a true source of entropy to seed the whole process, that is the hard part. See the Wikipedia entry on entropy for some starting points on techniques.

As an aside, if you did want cryptographically sequence random numbers from a ‘regular’ algorithm, a simple approach is to take a few random numbers in sequence, concatenate them together and then run something like MD5 or SHA-1 on them and the result is just as random and also cryptographically secure. That is, you just made your own ‘secure’ random number generator.