/// <summary>
/// Guess key
/// </summary>
/// <param name="ciphertext"></param>
/// <param name="keyLength"></param>
/// <returns></returns>
public static string GuessKey(string ciphertext, int keyLength, string language)
{
char[] key = new char[keyLength];
int numberLetters = Vigenere.LETTER_FREQUENCY[language].NumberOfLetters;
// METHOD 1 : Using language frequency for mutual indices of coincidence for all cipher vectors
for (int offset = 0; offset < keyLength; offset++)
{
double[] indices = new double[numberLetters];
double[] frequencies = Vigenere.Frenquencies(ciphertext, offset, keyLength).Select(Convert.ToDouble).ToArray();
indices = Vigenere.MutualIndicesOfCoincidence(Vigenere.LETTER_FREQUENCY[language].LetterFrequencies, 1, frequencies, frequencies.Sum());
int bestShift = 0;
for (int shift = 1; shift < numberLetters; shift++)
{
if (indices[shift] > indices[bestShift])
{
bestShift = shift;
}
}
key[offset] = (char)(bestShift + 65);
}
// METHOD 2 : Using reference vector for mutual indices of coincidence for all other cipher vectors
/*double[] frequenciesLang = Vigenere.LETTER_FREQUENCY[language].LetterFrequencies;
* double[] frequenciesRef = Vigenere.Frenquencies(ciphertext, 0, keyLength).Select(Convert.ToDouble).ToArray();
* double lengthRef = frequenciesRef.Sum();
*
* double[] indices = new double[numberLetters];
* indices = Vigenere.MutualIndicesOfCoincidence(frequenciesLang, 1, frequenciesRef, lengthRef);
*
* int shiftRef = 0;
* for (int i = 1; i < numberLetters; i++)
* if (indices[i] > indices[shiftRef])
* shiftRef = i;
*
* key[0] = (char)(shiftRef + 65);
*
* for (int offset = 1; offset < keyLength; offset++)
* {
* double[] frequenciesCmp = Vigenere.Frenquencies(ciphertext, offset, keyLength).Select(Convert.ToDouble).ToArray();
* double lengthCmp = frequenciesRef.Sum();
*
* indices = Vigenere.MutualIndicesOfCoincidence(frequenciesRef, lengthRef, frequenciesCmp, lengthCmp);
*
* int bestShift = 0;
* for (int i = 1; i < numberLetters; i++)
* if (indices[i] > indices[bestShift])
* bestShift = i;
*
* key[offset] = (char)((bestShift + shiftRef) % numberLetters + 65);
* }*/
return(new string(key));
}
/// <summary>
/// Calculate index of coincidence
/// </summary>
/// <param name="ciphertext"></param>
/// <param name="offset"></param>
/// <param name="keyLength"></param>
/// <returns></returns>
private static double IndexOfCoincidence(string ciphertext, int offset, int keyLength, int numberLetters = 26)
{
double index = 0;
int length = 0;
int[] frequences = Vigenere.Frenquencies(ciphertext, offset, keyLength);
for (int k = 0; k < numberLetters; k++)
{
index += frequences[k] * (frequences[k] - 1);
length += frequences[k];
}
return(index / (length * (length - 1)));
}