A secure alternative to System.Random

SecureRandom.cs

class SecureRandom : RandomNumberGenerator
{
    private readonly RandomNumberGenerator rng;

    public SecureRandom()
    {
        this.rng = new RNGCryptoServiceProvider();
    }
    public int Next()
    {
        var data = new byte[sizeof(int)];
        rng.GetBytes(data);
        return BitConverter.ToInt32(data, 0) & (int.MaxValue - 1);
    }
    public SecureString String(int len, string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890")
    {
        SecureString sb = new SecureString();
        string letter = string.Empty;
        while (sb.Length != len)
        {
            while (string.IsNullOrEmpty(letter) || !charset.Contains(letter))
            {
                if (sb.Length == len)
                {
                    sb.MakeReadOnly();
                    return sb;
                }
                byte[] oneByte = new byte[1];
                rng.GetBytes(oneByte);
                char c = (char)oneByte[0];
                if (char.IsDigit(c) || char.IsLetter(c))
                    letter = c.ToString();
            }
            sb.AppendChar(letter[0]);
            letter = string.Empty;
        }
        sb.MakeReadOnly();
        return sb;
    }
    public int Next(int maxValue)
    {
        return Next(0, maxValue);
    }
    public int Next(int minValue, int maxValue)
    {
        return (int)Math.Floor((minValue + ((double)maxValue - minValue) * NextDouble()));
    }

    public double NextDouble()
    {
        var data = new byte[sizeof(uint)];
        rng.GetBytes(data);
        var randUint = BitConverter.ToUInt32(data, 0);
        return randUint / (uint.MaxValue + 1.0);
    }
    public double NextDouble(double minimum, double maximum)
    {
        return NextDouble() * (maximum - minimum) + minimum;
    }
    public override void GetBytes(byte[] data)
    {
        rng.GetBytes(data);
    }
    public override void GetNonZeroBytes(byte[] data)
    {
        rng.GetNonZeroBytes(data);
    }
}