Question

I am developing a program in which I need to generate a random number from an array of bytes which is received from a device which generates random numbers. The problem is; the device sends the random numbers as a stream of bytes, and I need it to generate an integer between a certain value. In other words:

int GenerateRandom(int min, int max, byte[] rndr) {
//Do something with the bytes to make a random integer here between min and max
}

No, I can’t use the built-in Random class in .NET, because the program will be used to encrypt secure information, and a pseudo-random-number generator would not be acceptable.

Answer 1

The source code for the java.util.Random class may help you. I know, I know, it’s not C#, but don’t downvote just yet. The algorithm is the important bit: given a source of random bits, generate an integer between 0 and n. It’s pretty trivial to convert the range [0..n] to [min..max].

I’m sure you can implement the same algorithm in C# in a similar amount of code: around 12 lines, including 2 close braces and 2 lines of input validation.

EDIT: The nextInt(int n) function is the one you want to look at.

EDIT2: Alternatively, you could use a RNGCryptoServiceProvider seeded with your high-quality random bits. That may be secure enough for your purposes, especially if you can reseed every so often with new, high-quality randomness.

Interestingly, the msdn website does not work correctly with Chrome on Linux. Who’d have thunk?