code: def b64decode_url(istr):
      
      istr = encode_safe(istr)
      try:
          return urlsafe_b64decode(istr + '=' * (4 - (len(istr) % 4)))
      except TypeError as e:
          raise Error('Unable to decode base64: %s' % (e))
docs: JWT Tokens may be truncated without the usual trailing padding '='
          symbols. Compensate by padding to the nearest 4 bytes.