gpg --keyserver pgpkeys.mit.edu --recv-key 010908312D230C5F gpg -a --export 010908312D230C5F | sudo apt-key add - sudo apt-get update The problem should now be fixed. If you are missing multiple keys, ...

To make sure Apt can use the key, you need to configure the repository to know where it is. A repository configuration for our Opera example would look something like: deb [signed-by=/usr/share ...