Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  linux-generic linux-headers-generic linux-image-generic
The following packages will be upgraded:
  libcgmanager0 screen
2 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
Need to get 567 kB of archives.
After this operation, 67.6 kB of additional disk space will be used.
WARNING: The following packages cannot be authenticated!
  libcgmanager0 screen
E: There are problems and -y was used without --force-yes
Error: apt-get upgrade failed