public void GetNthSubkeysLetters_Test()
{
var result1 = Kasiski.GetNthSubkeysLetters(1, 3, "ABCABCABC");
Assert.AreEqual("AAA", result1);
var result2 = Kasiski.GetNthSubkeysLetters(2, 3, "ABCABCABC");
Assert.AreEqual("BBB", result2);
var result3 = Kasiski.GetNthSubkeysLetters(3, 3, "ABCABCABC");
Assert.AreEqual("CCC", result3);
var result4 = Kasiski.GetNthSubkeysLetters(1, 5, "ABCDEFGHI");
Assert.AreEqual("AF", result4);
var result5 = Kasiski.GetNthSubkeysLetters(1, 3, "ABC ABC ABC");
Assert.AreEqual("AAA", result5);
}
/// <summary>
/// Performs a Kasiski attack provided a given key length
/// </summary>
/// <param name="cipherText"></param>
/// <param name="keyLength"></param>
/// <returns></returns>
public static string AttackWithKeyLength(string cipherText, int keyLength)
{
var cipher = new Vigenere();
var allFreqScores = new List <Dictionary <char, int> >();
for (var i = 1; i < keyLength + 1; i++)
{
var nthLetters = Kasiski.GetNthSubkeysLetters(i, keyLength, cipherText.ToUpper());
var frequencyScore = new Dictionary <char, int>();
foreach (var letter in Constants.Letters)
{
var decryptedText = cipher.Decipher(nthLetters, Char.ToString(letter));
var score = Frequency.EnglishFreqMatchScore(decryptedText);
frequencyScore.Add(letter, score);
}
//Sort the dictionary by factor ocurrences descending
var sortedCommonFactors = (from entry in frequencyScore orderby entry.Value descending select entry)
.Take(4)
.ToDictionary(pair => pair.Key, pair => pair.Value);
allFreqScores.Add(sortedCommonFactors);
}
//Print our possible keys
var position = 1;
var possibleKeys = new List <char[]>();
foreach (var freq in allFreqScores)
{
_log.InfoFormat("Possible letters for letter {0} of the key: ", position);
var possibleKeyOutput = new StringBuilder();
var keyStore = new char[freq.Keys.Count];
var keyPosition = 0;
foreach (var keyPossible in freq.Keys)
{
possibleKeyOutput.AppendFormat("{0}\t", keyPossible);
keyStore[keyPosition] = keyPossible;
keyPosition++;
}
possibleKeys.Add(keyStore);
_log.InfoFormat(possibleKeyOutput.ToString());
position++;
}
var keyCombinations = Combinator.Combinations(possibleKeys);
var keyFound = false;
var key = string.Empty;
//Go through the possible keys with the given freqency and determine if they produce
//an English sentence.
Parallel.ForEach(keyCombinations, (possibleKey, loopState) =>
{
//Stop processing if we find the possible key
if (keyFound)
{
loopState.Break();
}
_log.DebugFormat("Attempting with key: {0}", possibleKey);
var decryptAttempt = cipher.Decipher(cipherText, possibleKey.ToString());
if (!English.IsEnglish(decryptAttempt))
{
return;
}
_log.InfoFormat("Found Possible Decryption Key: {0}", possibleKey);
keyFound = true;
key = possibleKey.ToString();
}
);
return(key);
}