C# 5.0 Programmer's Reference

Chapter 27 ❘ 729
is decrypting, the method subtracts a random value between 0 and 25 from each character.
It takes the new value modulo 26 and converts the result back into a letter.
The oddest trick here is that the code adds 26 to the character’s number if it is decoding. For
example, if the ciphertext letter is B = 1 and the random value subtracted is 10, then 1 – 10 =
–9. In C# the modulus operator % doesn’t care if its result is negative, so it leaves this value –9,
which does not convert back into a letter. The code adds 26, so the program calculates (–9 +
26) % 26 = 17 % 26 = 17, which converts to the letter R.
2. The BreakRandomCipher example program does this. It’s mostly straightforward, so download
the example to see how it works. The following code shows the trickiest part, which
uses LINQ to find the largest letter frequency.
// Get the letter frequencies.
var query =
from char ch in plaintext
group ch by ch into g
select g.Count();
// See how big the largest frequency is.
if (query.Max() / (float)plaintext.Length > maxFrequency)
{
// Hopefully this is it.
...
}
This code uses LINQ to get the letter counts in the plaintext. It then divides the largest
count by the number of letters in the message and compares that to the test frequency.
3. This approach doesn’t work well for short messages because the letter frequencies in short
messages may not match normal language usage. For example, in English the letter E appears
most often, but it doesn’t appear at all in the message “All good plans will bring victory!”
4. You could look at a measure of the distribution of the letters in the plaintext to see if it matches
what you expect for a correct decryption. For example, you could see if the top three letters
have frequencies around 12 percent, 9 percent, and 8 percent, the frequencies of the letters
E, T, and A in English, respectively. Or you could see if the standard deviations of the letters
are large. Unfortunately, both of those techniques are relatively time-consuming. The method
described in the exercise is less reliable, so it won’t work well with short messages, but it is
much faster.
You might make the test a bit more reliable if you look at the largest and smallest frequencies.
5. This program looks only at relative letter frequencies, so it should work with any language
where messages have uneven frequency distributions. The biggest change would be to allow
the program to work with characters that don’t lie between A and Z such as ö and ß.
6. The RandomIntegers example program uses the following code to provide the GetInt method.
public static class MyRandom
{
// The RNG.
private static RNGCryptoServiceProvider Rand = new RNGCryptoServiceProvider();
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