当我尝试安装或更新任何软件包时，我得到：
Untrusted packages could compromise your system's security.
You should only proceed with the installation if you are certain that
this is what you want to do.
我强烈怀疑这与我在更新时遇到的错误有关：
$ sudo aptitude update
Get: 1 http://ftp.us.debian.org wheezy InRelease [208 kB]
Get: 2 http://debian.lcs.mit.edu wheezy InRelease [208 kB]
Ign http://ftp.us.debian.org wheezy InRelease
Hit http://ftp.us.debian.org wheezy/main amd64 Packages/DiffIndex
Hit http://ftp.us.debian.org wheezy/contrib amd64 Packages/DiffIndex
Hit http://ftp.us.debian.org wheezy/non-free amd64 Packages/DiffIndex
Hit http://ftp.us.debian.org wheezy/contrib Translation-en
Hit http://ftp.us.debian.org wheezy/main Translation-en
Hit http://ftp.us.debian.org wheezy/non-free Translation-en
Get: 3 http://debian.lcs.mit.edu …