/// <summary>
/// Friedman test
/// </summary>
/// <param name="ciphertext"></param>
/// <param name="language"></param>
/// <param name="results"></param>
/// <param name="maxKeyLength"></param>
/// <returns></returns>
public static int FriedmanTest(string ciphertext, string language, out Dictionary <int, List <double> > results, int maxKeyLength = 16)
{
results = new Dictionary <int, List <double> >();
double languageIdc = Vigenere.LETTER_FREQUENCY[language].IndexOfCoincidence;
double average = 0;
double probableKeyAverage = 0;
int probableKeyLength = 0;
for (int keyLength = 2; keyLength <= maxKeyLength; keyLength++)
{
average = 0;
results[keyLength] = new List <double>();
for (int offset = 0; offset < keyLength; offset++)
{
double idc = Vigenere.IndexOfCoincidence(ciphertext, offset, keyLength);
results[keyLength].Add(idc);
average += idc;
}
average = average / keyLength;
if (Math.Abs(languageIdc - average) < Math.Abs(languageIdc - probableKeyAverage))
{
probableKeyLength = keyLength;
probableKeyAverage = average;
}
}
return(probableKeyLength);
}
private void btnRunFriedman_Click(object sender, EventArgs e)
{
if (this.txtCiphertext.Text == string.Empty)
{
MessageBox.Show(this, "The ciphertext message must be defined!", "Error");
}
else
{
string ciphertext = this.txtCiphertext.Text.Replace(" ", string.Empty).Replace("\n", string.Empty).Replace("\r", string.Empty);
StringBuilder sb = new StringBuilder();
Dictionary <int, List <double> > friedmanResults;
int probableKeyLength = Vigenere.FriedmanTest(ciphertext, this.cbbLanguage.Text, out friedmanResults, (int)this.nudMaxKeyLength.Value);
sb.Append("Key length : IC(x)\r\n");
foreach (KeyValuePair <int, List <double> > friedmanResult in friedmanResults)
{
sb.Append(friedmanResult.Key + " :");
foreach (double value in friedmanResult.Value)
{
sb.Append(" " + Math.Round(value, 3));
}
sb.Append("\r\n");
}
this.txtResultFriedman.Text = sb.ToString();
this.txtProbableKeyLength.Text = probableKeyLength.ToString();
this.nudAnalyseFrequencyKeyLength.Value = probableKeyLength;
}
}
/// <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));
}
private void btnAnalyseFrequency_Click(object sender, EventArgs e)
{
if (this.txtCiphertext.Text == string.Empty)
{
MessageBox.Show(this, "The ciphertext message must be defined!", "Error");
}
else
{
string ciphertext = this.txtCiphertext.Text.Replace(" ", string.Empty).Replace("\n", string.Empty).Replace("\r", string.Empty);
this.txtAnalyseFrequenciesKey.Text = Vigenere.GuessKey(ciphertext, (int)this.nudAnalyseFrequencyKeyLength.Value, this.cbbLanguage.Text);
this.txtKey.Text = this.txtAnalyseFrequenciesKey.Text;
}
}
/// <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)));
}
private void btnDecrypt_Click(object sender, EventArgs e)
{
if (this.txtCiphertext.Text == string.Empty)
{
MessageBox.Show(this, "The ciphertext message must be defined!", "Error");
}
else if (this.txtKey.Text == string.Empty)
{
MessageBox.Show(this, "The key must be defined!", "Error");
}
else
{
this.txtCleartext.Text = Vigenere.Decrypt(this.txtCiphertext.Text, this.txtKey.Text);
}
}
/// <summary>
/// Decrypt the ciphertext message
/// </summary>
/// <param name="ciphertext"></param>
/// <param name="key"></param>
/// <returns></returns>
public static string Decrypt(string ciphertext, string key)
{
return(Vigenere.ProcessKey(ciphertext, key, -1));
}
/// <summary>
/// Encrypt the cleartext message
/// </summary>
/// <param name="cleartext"></param>
/// <param name="key"></param>
/// <returns></returns>
public static string Encrypt(string cleartext, string key)
{
return(Vigenere.ProcessKey(cleartext, key, 1));
}