If you get the problem above with Synaptic, enter the following commands in a terminal:
gpg --keyserver keyserver.ubuntu.com --recv 2836CB0A8AC93F7A gpg --export --armor 2836CB0A8AC93F7A | sudo apt-key add -
Repeat for every key missing, changing 2836CB0A8AC93F7A with whatever you see in the error dialog box. After doing this, you can click on the reload button in Synaptic.
