code: def parse_base64(base64_string):
    
  
    if not isinstance(base64_string, six.string_types):
      message = 'Invalid argument: '+repr(base64_string)
      raise securesystemslib.exceptions.FormatError(message)
  
    extra = len(base64_string) % 4
    if extra:
      padding = '=' * (4 - extra)
      base64_string = base64_string + padding
  
    try:
      return binascii.a2b_base64(base64_string.encode('utf-8'))
  
    except (TypeError, binascii.Error) as e:
      raise securesystemslib.exceptions.FormatError('Invalid base64'
        ' encoding: ' + str(e))
docs: <Purpose>
      Parse a base64 encoding with whitespace and '=' signs omitted.
  
    <Arguments>
      base64_string:
        A string holding a base64 value.
  
    <Exceptions>
      securesystemslib.exceptions.FormatError, if 'base64_string' cannot be parsed
      due to an invalid base64 encoding.
  
    <Side Effects>
      None.
  
    <Returns>
      A byte string representing the parsed based64 encoding of
      'base64_string'.