16.04 update center : untrusted packages issue

Question

This is a question from a French user of the 16.04. I have been unable to keep my computer up to date for about six months I think. For some reason I do not know, the update center had ceased to work on an automatic basis. It did not alarm me for awhile, but now, due to some dated hardware material and what seems to be a general lack of compatibility with firefox, it's become quite painful to use Ubuntu.

So when I started using the update center again, I was quickly confronted to this "requires the installation of untrusted packages". I checked several topics dealing with this problem and got kind of confused noticing all the different typologies of problems and causes. I tried a few options, if not all of them :

• choosing the "best server" in the software and updates section => didn't work
• choosing server for France and main server => didn't work either
• running the terminal and enter these commands, as advised by user Maythux : sudo apt-get clean sudo mv /var/lib/apt/lists /var/lib/apt/lists.bak sudo mkdir -p /var/lib/apt/lists/partial sudo apt-get clean sudo apt-get update

=> I took captures of the terminal's conclusions. Download center was set on the "best server". For I am not able to translate this all by myself, I'm posting the pics right here. I'm new here and I cannot post more than two links, so I'll put the final two. Hope it helps. If it doesn't, I guess I can send you the rest by private message.

[SPOILER] [IMG]http://i66.tinypic.com/2hodawl.jpg[/IMG] [/SPOILER]

[SPOILER] [IMG]http://i67.tinypic.com/30tl1c3.jpg[/IMG] [/SPOILER]

Thank you guies. BZHbutter