private static int DetermineKeyLength(string ciphertext)
        {
            double difference = double.MaxValue;
            int keyLength = 0;

            for (int i = 1; i < MAXKEYSIZE; i++)
            {
                if (i >= ciphertext.Length) break;
                double current = IndexOFCoincidence(ciphertext, i) - EXPECTEDENGLISHIOC;
                if (current < difference)
                {
                    difference = current;
                    keyLength = i;
                }
            }
            return keyLength;
        }

        public static double IndexOFCoincidence(string ciphertext, int shift)
        {
            double ioc = 0;
            for (int i = 0; i < ciphertext.Length; i++)
            {
                if (ciphertext[i] == ciphertext[(i + shift) % ciphertext.Length])
                {
                    ioc++;
                }
            }

            return ioc/ciphertext.Length;
        }