Force update from unsigned repository
You can set options in your sources.list
(located at /etc/apt/sources.list
):
deb [trusted=yes] http://www.deb-multimedia.org jessie main
The trusted option is what turns off the GPG check. See man 5 sources.list
for details.
You can either edit the file within the terminal with vim ( or whatever you prefer) or any non-terminal editor like gedit.
You can bypass some important safeguards by using the following option:
--allow-unauthenticated
From the man pages for apt-get:
--allow-unauthenticated
Ignore if packages can't be authenticated and don't prompt about
it. This can be useful while working with local repositories, but
is a huge security risk if data authenticity isn't ensured in
another way by the user itself. The usage of the Trusted option for
sources.list(5) entries should usually be preferred over this
global override. Configuration Item:
APT::Get::AllowUnauthenticated.
But be a little cautious about using this option more widely, the safeguards are in place to protect your computer not limit your freedom...
Another generic solution would be
sudo apt-key adv --keyserver pgp.mit.edu --recv-keys 5C808C2B65558117
Note: I didn't test the solution with this repository but I did it with Skype repository and it worked just fine.
Another solution specific to your case is to install the keys
wget http://www.deb-multimedia.org/pool/main/d/deb-multimedia-keyring/deb-multimedia-keyring_2012.05.05_all.deb -O deb-multimedia-keyring.deb
sudo dpkg -i multimedia-keyring_all.deb
As described in the full walk through Here