@classmethod
def _obfusc50(cls, block):
    """Unscrambles blocks of 50 by character substitution

        :param block: A string of 50 characters
        :return: An unscrambled string
        """
    result = list(block)
    for i in range(5):
        result[i] = block[49 - i]
        result[49 - i] = block[i]
    for i in range(10, 24):
        result[i] = block[49 - i]
        result[49 - i] = block[i]
    return ''.join(result)
