Encodes a username and password for the EXTERNAL mechanism. Since authentication is handled by an encapsulating protocol like SSL or UNIX domain sockets, EXTERNAL doesn‘t pass along any username or password information at all and this method always returns the empty string.

@param [String] username The username to encode. This parameter is

  ignored.

@param [String] password The password to encode. This parameter is

  ignored.

@return [String] The username and password, encoded for the

  EXTERNAL mechanism. This is always the empty string.