C# 5.0 Programmer's Reference

Randomness ❘ 605
A 32-bit integer can hold 2 32 (4.3 billion) different values, so there are approximately 4.3 billion ways
you can initialize a Random object. To think of it another way, there are roughly 4.3 billion possible
keys. That may seem like a lot of keys, but to a computer it’s actually not many. It wouldn’t be hard to
write a program that tries every possible seed value.
For each seed value, the program would try to decrypt the message. If the result looks like English (or
whatever language you use), the seed is probably correct. For example, in English some letters such
as E appear more often than others. If the seed is correct, some letters should appear more often than
others. If the seed is incorrect, all the letters should appear with roughly the same frequency.
Generating Random Numbers
Generating cryptographically strong random numbers is actually fairly easy in C#. Simply create a
cryptographic random number generator object and use its methods to generate the numbers. The
.NET Framework’s RNGCryptoServiceProvider class provides this kind of random number generator.
(The “RNG” at the beginning of the class’s name stands for “random number generator.”)
The RandomNumbers example program, which is available for download on the book’s website,
uses the following code to generate a list of random numbers.
// Generate random numbers.
private void generateButton_Click(object sender, EventArgs e)
{
numbersListBox.Items.Clear();
int numNumbers = int.Parse(numNumbersTextBox.Text);
// Make the RNG.
RNGCryptoServiceProvider rand = new RNGCryptoServiceProvider();
// Make a buffer to hold 4 random bytes.
byte[] bytes = new byte[4];
// Make the random numbers.
for (int i = 0; i < numNumbers; i++)
{
// Get 4 random bytes.
rand.GetBytes(bytes);
// Convert the bytes into an integer.
int number = BitConverter.ToInt32(bytes, 0);
}
}
// Display the number.
numbersListBox.Items.Add(number);
The program starts by clearing its output ListBox and parsing the number of random numbers it
should generate.
The code then creates an RNGCryptoServiceProvider object. That object generates random bytes
but this program needs to display random numbers. To convert bytes into numbers, the program
creates a byte array large enough to hold the number of bytes it needs. This program generates int
values, which are 4-byte integers, so it makes an array holding 4 bytes.
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