code: def aes_key(self, data):
          
          old = self.key
          if data:
              new = self._decode_input_string(data)
              if len(new) == 16:
                  self.key = new
              else:
                  raise yubico_exception.InputError('AES128 key must be exactly 16 bytes')
  
          return old
docs: AES128 key to program into YubiKey.
  
          Supply data as either a raw string, or a hexlified string prefixed by 'h:'.
          The result, after any hex decoding, must be 16 bytes.