15.04 update not possible to install

[Wolf Rettig]

system not possible to update
public key is not available

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-74.118-generic 3.13.11-ckt30
Uname: Linux 3.13.0-74-generic i686
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: i386
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Dec 21 17:02:53 2015
InstallationDate: Installed on 2013-01-03 (1081 days ago)
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release i386 (20121017.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2015-12-21 (0 days ago)
VarLogDistupgradeTermlog:

modified.conffile..etc.update.manager.release.upgrades: [modified]
mtime.conffile..etc.update.manager.release.upgrades: 2015-11-17T08:48:25.738688