Performs a Shannon entropy analysis on a given block of data.

def shannon(self, data):
        '''
        Performs a Shannon entropy analysis on a given block of data.
        '''
        entropy = 0

        if data:
            length = len(data)

            seen = dict(((chr(x), 0) for x in range(0, 256)))
            for byte in data:
                seen[byte] += 1

            for x in range(0, 256):
                p_x = float(seen[chr(x)]) / length
                if p_x > 0:
                    entropy -= p_x * math.log(p_x, 2)

        return (entropy / 8)