apt-get update 提示没有公钥，无法验证签名的解决办法

root@debian:~# apt-get update
命中 http://mirrors.163.com squeeze Release.gpg
忽略 http://mirrors.163.com/debian/ squeeze/contrib Translation-en
忽略 http://mirrors.163.com/debian/ squeeze/contrib Translation-zh
忽略 http://mirrors.163.com/debian/ squeeze/contrib Translation-zh_CN
忽略 http://mirrors.163.com/debian/ squeeze/main Translation-en
命中 http://mirrors.163.com/debian/ squeeze/main Translation-zh
命中 http://mirrors.163.com/debian/ squeeze/main Translation-zh_CN
忽略 http://mirrors.163.com/debian/ squeeze/non-free Translation-en
忽略 http://mirrors.163.com/debian/ squeeze/non-free Translation-zh
忽略 http://mirrors.163.com/debian/ squeeze/non-free Translation-zh_CN
获取：1 http://mirrors.163.com squeeze-proposed-updates Release.gpg [836 B]
命中 http://mirrors.163.com/debian/ squeeze-proposed-updates/contrib Translation-en
忽略 http://mirrors.163.com/debian/ squeeze-proposed-updates/contrib Translation-zh
忽略 http://mirrors.163.com/debian/ squeeze-proposed-updates/contrib Translation-zh_CN
命中 http://mirrors.163.com/debian/ squeeze-proposed-updates/main Translation-en
忽略 http://mirrors.163.com/debian/ squeeze-proposed-updates/main Translation-zh
忽略 http://mirrors.163.com/debian/ squeeze-proposed-updates/main Translation-zh_CN
命中 http://mirrors.163.com/debian/ squeeze-proposed-updates/non-free Translation-en
忽略 http://mirrors.163.com/debian/ squeeze-proposed-updates/non-free Translation-zh
忽略 http://mirrors.163.com/debian/ squeeze-proposed-updates/non-free Translation-zh_CN
命中 http://mirrors.163.com squeeze Release
获取：2 http://mirrors.163.com squeeze-proposed-updates Release [144 kB]
命中 http://mirrors.163.com squeeze/main Sources
忽略 http://mirrors.163.com squeeze-proposed-updates Release
命中 http://mirrors.163.com squeeze/non-free Sources
命中 http://mirrors.163.com squeeze/contrib Sources
命中 http://mirrors.163.com squeeze/main amd64 Packages
命中 http://mirrors.163.com squeeze/non-free amd64 Packages
命中 http://mirrors.163.com squeeze/contrib amd64 Packages
命中 http://mirrors.163.com squeeze-proposed-updates/main Sources/DiffIndex
命中 http://mirrors.163.com squeeze-proposed-updates/non-free Sources/DiffIndex
命中 http://mirrors.163.com squeeze-proposed-updates/contrib Sources/DiffIndex
命中 http://mirrors.163.com squeeze-proposed-updates/main amd64 Packages/DiffIndex
命中 http://mirrors.163.com squeeze-proposed-updates/non-free amd64 Packages/DiffIndex
命中 http://mirrors.163.com squeeze-proposed-updates/contrib amd64 Packages/DiffIndex
下载 837 B，耗时 0秒 (1,610 B/s)
正在读取软件包列表... 完成
W: GPG 错误：http://mirrors.163.com squeeze-proposed-updates Release: 由于没有公钥，无法验证下列签名： NO_PUBKEY 8B48AD6246925553
root@debian:~# gpg --keyserver hkp://keyserver.ubuntu.com:11371 --recv-keys 46925553
gpg: 下载密钥‘46925553’，从 hkp 服务器 keyserver.ubuntu.com
gpg: 密钥 46925553：“Debian Archive Automatic Signing Key (7.0/wheezy) <ftpmaster@debian.org>”未改变
gpg: 合计被处理的数量：1
gpg: 未改变：1 下载公钥
root@debian:~# gpg --export --armor 46925553 | sudo apt-key add -
OK 导入公钥