我已经安装了 Debian wheezy。然后我想设置apt源，如下所示：
deb http://security.debian.org/ wheezy/updates main contrib
deb-src http://security.debian.org/ wheezy/updates main contrib
deb http://ftp.debian.org/debian/ wheezy-updates main contrib
deb-src http://ftp.debian.org/debian/ wheezy-updates main contrib
然后我执行命令'sudo apt-get update'，我收到错误消息：
Get:1 http://security.debian.org wheezy/updates Release.gpg
Get:2 http://security.debian.org wheezy/updates Release
Ign http://security.debian.org wheezy/updates Release
E: GPG error: http://security.debian.org wheezy/updates Release: The following signatures were invalid: NODATA 1 NODATA 2
我的 apt-key 列表输出是：
/etc/apt/trusted.gpg.d//debian-archive-squeeze-automatic.gpg
------------------------------------------------------------
pub 4096R/473041FA 2010-08-27 [expires: 2018-03-05]
uid Debian Archive Automatic Signing Key (6.0/squeeze) <ftpmaster@debian.org>
/etc/apt/trusted.gpg.d//debian-archive-squeeze-stable.gpg
---------------------------------------------------------
pub 4096R/B98321F9 2010-08-07 [expires: 2017-08-05]
uid Squeeze Stable Release Key <debian-release@lists.debian.org>
/etc/apt/trusted.gpg.d//debian-archive-wheezy-automatic.gpg
-----------------------------------------------------------
pub 4096R/46925553 2012-04-27 [expires: 2020-04-25]
uid Debian Archive Automatic Signing Key (7.0/wheezy) <ftpmaster@debian.org>
/etc/apt/trusted.gpg.d//debian-archive-wheezy-stable.gpg
--------------------------------------------------------
pub 4096R/65FFB764 2012-05-08 [expires: 2019-05-07]
uid Wheezy Stable Release Key <debian-release@lists.debian.org>
如何解决 GPG 错误？