So you run your apt-get update after adding a PPA or other repository, and you come across a warning like this:
W: GPG error: http://toolbelt.heroku.com ./ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C927EBE00F1B0520 W: GPG error: http://ftp.osuosl.org wheezy Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY CBCB082A1BB943DB W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D46F45428842CE5E
While these are just warnings, and can be ignored, doing such is irresponsible as it means that apt cannot verify the packages it downloads as being signed by the authors. Most repositories give instructions on how to download and import the PGP key, but not all do. So here’s a foolproof method of obtaining the key and importing it in to apt.
For this example we’ll use the last one, which is the key for the official Bitcoin client PPA for Ubuntu. It (the Precise Pangolin branch) also works fine under Debian Wheezy.
The first step is to get the key in to your keyring.
root@debian:~# gpg --keyserver hkp://subkeys.pgp.net --recv-keys D46F45428842CE5E gpg: directory `/root/.gnupg' created gpg: new configuration file `/root/.gnupg/gpg.conf' created gpg: WARNING: options in `/root/.gnupg/gpg.conf' are not yet active during this run gpg: keyring `/root/.gnupg/secring.gpg' created gpg: keyring `/root/.gnupg/pubring.gpg' created gpg: requesting key 8842CE5E from hkp server subkeys.pgp.net gpg: /root/.gnupg/trustdb.gpg: trustdb created gpg: key 8842CE5E: public key "Launchpad PPA for Bitcoin" imported gpg: no ultimately trusted keys found gpg: Total number processed: 1 gpg: imported: 1 (RSA: 1)
The first several lines will only show if this is your first time doing anything PGP related, and are of no concern to us. The rest of the output shows that we received the key from the keyserver, however it isn’t marked as trusted by us. Marking it as trusted is optional and provides no actual benefit, unless you are able to personally verify that the key is correct (as in, you walked up to the key maintainer and exchanged keys in person, or some other highly secure method of verification). The important part is that you are now protected from man-in-the-middle attacks, so long as they signing key isn’t compromised.
Now we need to add them to the apt’s key store. This is very easy.
root@debian:~# gpg --export --armor D46F45428842CE5E | apt-key add -- OK
Basically we export the key from GnuPG in a plain text format supported by apt-key, to stdout, then have apt-key read the key from stdin.
Do this entire process for each missing public key, and the warnings will be fixed and your system will be secured against a potential attack vector.