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 ...

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, ...