zipFiles(source, out) {
		const archive = archiver('zip', { zlib: { level: 9 } });
		const stream  = fs.createWriteStream(out);

		return new Promise((resolve, reject) => {
			source
				.map((fl) => {
					let fileName = path.basename(fl);
					logger.debug(fl, 'added to certificate zip');
					archive.file(fl, { name: fileName });
				});
			archive
				.on('error', (err) => reject(err))
				.pipe(stream);

			stream.on('close', () => resolve());
			archive.finalize();
		});
	}
@param   {String}  source
@param   {String}  out
@returns {Promise}