public static int FindBlockSizeWithCoincidence(string cipherText, int maximumBlockSize = Constants.MAX_BLOCK_SIZE)
{
// Try different block sizes starting from 2 to maximumBlockSize.
int currentBlockSize = 2;
// Store average error calculated between the found coincidence and English language's coincidence in a dictionary.
Dictionary <int, double> errorsPerBlockSize = new Dictionary <int, double>();
while (currentBlockSize <= maximumBlockSize)
{
double error = 0;
// Divide cipher text according to the block size.
IEnumerable <string> dividedCipherTexts = DivideCipherText(cipherText, currentBlockSize);
// For each divided cipher text, calculate coincidence and obtain error.
for (int i = 0; i < currentBlockSize; i++)
{
double coincidence = Coincidence.CalculateIndex(dividedCipherTexts.ElementAt(i));
error += Math.Abs(coincidence - Constants.EnglishIndexOfCoincidence);
// Check after every 5 calculation of index of coincidence on a divided cipher text to see whether
// current block size is yielding good results. If error is too large, try next block size.
if (i % 5 == 0)
{
double currentError = error / currentBlockSize;
if (currentError > 0.01)
{
break;
}
}
}
// Take average of the calculated error sum.
error /= currentBlockSize;
// If error is less than %0.5 (if average coincidence is between 0.060 and 0.070), store as a candidate block size.
if (error < 0.005)
{
errorsPerBlockSize.Add(currentBlockSize, error);
}
// Increment the block size and try again.
currentBlockSize++;
}
if (errorsPerBlockSize.Count == 0)
{
// If no suitable block size has been found, return -1 to signify the analysis has failed.
return(-1);
}
else
{
// Get the block size that resulted in the lowest error and return it.
errorsPerBlockSize = errorsPerBlockSize.OrderBy(k => k.Value).ToDictionary(k => k.Key, v => v.Value);
return(errorsPerBlockSize.First().Key);
}
}
