Comment 2 for bug 277677

This is not a bug, you have to install the PGP key from medibuntu in order to make this work.

Open a terminal and copy and paste this whole line :

wget -q http://fr.packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add -

hit enter and try again to update your packages list with synaptic (or apt-get).