Additional keyrings for APT in trusted.gpg.d

Keys downloaded from repositories should be joint into a new GPG keyring so you can drop them into '/etc/apt/trusted.gpg.d/*.gpg', like this:

gpg --no-default-keyring --keyring ./Opera.gpg --import Opera.key
sudo cp Opera.gpg /etc/apt/trusted.gpg.d/Opera.gpg

You can actually get the best of both worlds: create an additional keyring in /etc/apt/trusted.gpg.d/ and use apt-key instead of gpg directly.

If you already have a keyfile locally, such as Opera.key, then run the following command:

sudo apt-key --keyring /etc/apt/trusted.gpg.d/Opera.gpg add Opera.key

Of course, you can still import the key directly as MestreLion demonstrated:

wget -q -O - http://deb.opera.com/archive.key | sudo apt-key --keyring /etc/apt/trusted.gpg.d/Opera.gpg add -

Additionally, you could use apt-key to add the key for you in instead of manually dropping a file to that path. Assuming you saved the file as Opera.key:

sudo apt-key add Opera.key

You could even download and import the key file on-the-fly, instead of saving it to a local file:

wget -q -O - http://deb.opera.com/archive.key | sudo apt-key add -

Apt-key manages the contents of /etc/apt/trusted.gpg main file instead of using the directory, which may be a convenience or a burden for you.