How to create a cryptographically secure random integer within a range?

Question

I need to create a single safe random integer in a given range for a program that generates passwords. Now I use this:

RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
byte[] rand = new byte[4];
rng.GetBytes(rand);
int i = BitConverter.ToUInt16(rand, 0);
int result = i%max;   //max is the range upper bound (the lower is 0)

Is this method safe for cryptographic purposes? If not, how do I do this?

Answer 1

You can take a look at the CryptoRandom class, taken from https://gist.github.com/1017834 , which is the original version of Stephen Tuub and Sean Farkas. In this class, they implement several random generators that appear to be cryptographically fixed.

int.

public class RandomGenerator
{
    readonly RNGCryptoServiceProvider csp;

    public RandomGenerator()
    {
        csp = new RNGCryptoServiceProvider();
    }

    public int Next(int minValue, int maxExclusiveValue)
    {
        if (minValue >= maxExclusiveValue)
            throw new ArgumentOutOfRangeException("minValue must be lower than maxExclusiveValue");

        long diff = (long)maxExclusiveValue - minValue;
        long upperBound = uint.MaxValue / diff * diff;

        uint ui;
        do
        {
            ui = GetRandomUInt();
        } while (ui >= upperBound);
        return (int)(minValue + (ui % diff));
    }

    private uint GetRandomUInt()
    {
        var randomBytes = GenerateRandomBytes(sizeof(uint));
        return BitConverter.ToUInt32(randomBytes, 0);
    }

    private byte[] GenerateRandomBytes(int bytesNumber)
    {
        byte[] buffer = new byte[bytesNumber];
        csp.GetBytes(buffer);
        return buffer;
    }
}