Calculates the entropy of a string based on known frequency of
        English letters.

        Args:
            entropy_string: A str representing the string to calculate.

        Returns:
            A negative float with the total entropy of the string (higher
            is better).

def calculate_entropy(self, entropy_string):
        """Calculates the entropy of a string based on known frequency of
        English letters.

        Args:
            entropy_string: A str representing the string to calculate.

        Returns:
            A negative float with the total entropy of the string (higher
            is better).
        """
        total = 0
        for char in entropy_string:
            if char.isalpha():
                prob = self.frequency[char.lower()]
                total += - math.log(prob) / math.log(2)
        logging.debug("Entropy score: {0}".format(total))
        return total