我在 Ubuntu 16.04 中使用来自 Debian 多媒体的未签名存储库：
deb http://www.deb-multimedia.org jessie main
要安装deb-multimedia-keyring，我正在运行：
apt-get update && apt-get install deb-multimedia-keyring -y
这给出了一个错误：
W: GPG error: http://www.deb-multimedia.org jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5C808C2B65558117
E: The repository 'http://www.deb-multimedia.org jessie InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.