最近更新镜像源列表时总提示GPG校验错误，看了下，原来是debian-multimedia这个源没有公钥，无法进行签名验证，登录http://www.multimedia.org，找到FAQ查看了，找到了解决的方法：
Apparently you have never noticed but my packages can be checked with apt-check-sigs only with apt-get 0.5.x as explained  here (chapter 7.4.3)

Method 1 (Simple, fast and easy) :
You can install (only in unstable and testing) the debian-multimedia-keyring package.

Method 2 :
Here is my fingerprint :
1D7F C53F 80F8 52C1 88F4 ED0B 07DC 563D 1F41 B907

You can import my key from a GPG server (needed for all case) :
gpg --keyserver hkp://wwwkeys.eu.pgp.net --recv-keys 1F41B907

Or you can download my key here (right click and Save Link As (wget doesn't work here)) and then add my key with the following command :
gpg --import < gpgkey.pub

When my key is added in your keyring do the following,

With apt-get >= 0.6 do the following (sudo is needed) :
gpg --armor --export 1F41B907 | sudo apt-key add -

With su (Thanks to Michael Gilbert) :

gpg --armor --export 1F41B907 > debian-multimedia.key
su -c "apt-key add debian-multimedia.key"

If you don't use sudo, do the following under root :
gpg --armor --export 1F41B907 | apt-key add -
                                ----linux-fox
                                  2007/03/03