If we add the repository http://www.debian-multimedia.org on Debian testing
it can happen that when we try to execute the comand apt-get update, we see a GPG error:

W: GPG error: http://www.debian-multimedia.org squeeze Release:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907

To solve this problem we need to install the debian-keyring package

# apt-get install debian-keyring

And then execute this command:

# gpg --keyring /usr/share/keyrings/debian-keyring.gpg -a --export 07DC563D1F41B907 |apt-key add -

After that, when we try to execute apt-get update, there will be no the GPG error anymore.

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