How to generate a random integer between certain values from an array of bytes

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开发者_Python百科 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.

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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?

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If you need a cryptograhically strong random number then the .NET framework has the RNGCryptoServiceProvider class for this purpose.

GetBytes() fills an array of bytes with a cryptographically strong sequence of random values.

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If this array of bytes already contains a random number, you can use it as a sample value. In this example I am using the first 4 bytes, you can modify this for whatever word size is applicable and make it rotate words.

static int GenerateRandom(int min, int max, byte[] rndr)
{
    int isamp = (int)(((uint)rndr[3] << 24) | ((uint)rndr[2] << 16) | ((uint)rndr[1] << 8) | ((uint)rndr[0]));
    if (isamp < 0)
        isamp += int.MaxValue;
    double samp = isamp * 4.6566128752457969E-10;
    return (int)(samp * (max - min)) + min;
}