WARNING: The following packages cannot be authenticated!

$ sudo apt-get install  gstreamer1.0-pulseaudio
Reading package lists… Done
Building dependency tree      
Reading state information… Done
The following packages were automatically installed and are no longer required:
  libdirac-encoder0 libopencore-amrnb0 libopencore-amrwb0 libsvga1 libvo-aacenc0 libvo-amrwbenc0
Use ‘apt-get autoremove’ to remove them.
The following NEW packages will be installed:
  gstreamer1.0-pulseaudio
0 upgraded, 1 newly installed, 0 to remove and 1 not upgraded.
Need to get 57.6 kB of archives.
After this operation, 220 kB of additional disk space will be used.
WARNING: The following packages cannot be authenticated!
  gstreamer1.0-pulseaudio
Install these packages without verification [y/N]?

To solve that:

<code>$ sudo apt-key update  $ sudo apt-get update  </code>

Related:

$ sudo apt-key list
$ sudo apt-key remove FBB75451
$ sudo apt-key list