Install requires installation of untrusted package?
This error is given when Software Center failed to download and install the package signing key for software you purchased. Normally, this is all automatic, but in case it fails you can still do it manually.
To manually install the package key...
Go here in a browser:
- Click on the title of the package you want.
Where it says OpenPGP key 1024R/XXXXXXXX, copy the 8 digits marked with an X. This is the short version of the key ID.
In a terminal, run this command:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys XXXXXXXX
Replace the XXXXXXXX with the key ID.
You will need to enter your password for this command to complete.
Afterward, the correct key should be installed and you should be able to get the software. Just use 'File -> Reinstall previous purchases' in Ubuntu Software Center.