public function makeTempFile( $prefix = '', $content = '' ) {
		$fileName = $prefix . $this->getRandomHexString();
		$filePath = $this->path . DIRECTORY_SEPARATOR . $fileName;
		$ok = file_put_contents( $filePath, $content );
		if ( $ok === false ) {
			throw new GpgLibException( "Could not create temporary file at $filePath" );
		}
		return $filePath;
	}